As a strong proponent of same-sex marriage, this might sound weird or difficult to believe at first, but I think I understand what you're asking here. I know several people who have (or have had) difficulties accepting equal marriage rights, in particular because they looked at those marriages from a religious perspective.

I respect your viewpoint. I don't agree with it, but I respect it and I don't hold it against you, because that's just what it is: it's your viewpoint. It's based on who you are, what you believe in and what you feel is right and what is not. However, a gouvernament is not a religious institution. To guarantee freedom of religion, it has to -by definition- ignore all religious arguments for or against any problem (problem meaning a social reality in need of change, or legislation). The main Abrahamic religions seem to all oppose same-sex marriage, but some cultures don't.
For a legislator to be able to guarantee all people the same freedoms and obligations, it can't accept religous arguments. That should be self evident.

However, on a human level, I do get why people such as yourself find it difficult to accept same-sex marriage, but as I previously tried to explain, the type of marriage we're talking about is a legal marriage. It's people expressing their commitment towards each other in accordance to a man-made law. No law can ever require you to find this a good thing, just as no law can force me to get angry at you for seeing things differently.

I guess what I'm trying to say here is that, as a Catholic, the best way forward would be to say that you understand why the Supreme Court really had no choice to rule any other way than the way it did. I'm not sure what/where, but I believe there's even something in the Bible about accepting the rules of the land where you live. In the US there's a Church-State separation after all. At the same time, it's perfectly OK for you to say that you don't agree with the ruling, on a personal level. Again: you're guaranteed freedom of speech and beliefs, which implies freedom of though.
You needn't look at this ruling as man-made laws rewriting the bible. I know that the Bible does not condone same-sex marriage, so when two men or women get married, it can't be a "Biblical marriage", it's a legal marriage. One that you, as a citizen must recognize. But it's not a marriage that the Catholic church recognizes at the moment.

Bottom line is, as a citizen, you'll have to tolerate same-sex marriage. As a Catholic, very little has changed. From a religious viewpoint, the marriages don't conform to the basic, scriptural, criteria.
Just find a way to make that distinction, maybe that can help you explain your position to outsiders.

After that, your question of "spreading the joy of traditional marriage" is actually part of your task as a Catholic of witnessing. Seeing as marriage is one of the 7 sacraments, it most definitely is normal for you to bring it up when witnessing.

The first encounter with faith that a Christian experience is the faithfulness of a husband to his wife, a wife to her husband, and a father and mother to their child. When you rip this apart, you make children have a hard time understanding what faith is. Think about it: Jesus even uses the term "Father" to describe the "maker of Heaven and earth, of all that is seen and unseen." If you don't understand human fatherhood, how can you possibly understand Divine Fatherhood? Remember, the Church is often described as the Family of God, with us being adopted sons and daughters too. How can a child with a hard home life understand this :(

There are political reasons as well (which are secondary in importance to the primary reasons above), like the fact that only cultures who see family roles as religious duties, as sacred, progress to become civilizations (and stay that way). A duty to the family and family values also keeps the government in check, because men are far more likely to fight for his wife, children and home than for abstractions and far-away harbors. As G. K. Chesterton puts it:

We have a vision of a god of roofs and a god of gate-posts, of a god of doors and even a god of drains. It has been suggested that all mythology was a sort of fairy-tale; but this was a particular which avenged an insult to a daughter. The truth is that only men to whom the family is sacred will ever have a standard or a status by which to criticize the state. They alone can appeal to something more holy than the gods of the city; the gods of the hearth.

G.K. Chesterton - The Everlasting Man - Chapter 7

One federal judge once explained that anyone who works in his field has seen the evidence of what happens to young, poor, uneducated people when family structures are undermined: he laments on such a cruel social order.

Remember, the modern world has gone so mad that Christians are no longer only defending Divine values, but rather simply humans ones; ones in which the ancient pagans easily understood. We not only defend the Holy Family, we now also defend the human family.

It is not only in a time like our own, but even at certain stages in a Christian's life, "the going gets tough". It is time for the Christian to remember that we are not to grow weary in well-doing, for in due season we shall reap, if we do not lose heart.

So for the laity, where can one find the teaching that can guide them to this well-doing?

One such teaching is the Post Synodal Apostolic Exhortation Christifideles Laici Of His Holiness Pope St. John Paul II On The Vocation And Mission Of The Lay Faithful In The Church And In The World. Some excerpts (my emphasis):

• The lay faithful are sharers in the priestly mission, for which Jesus offered himself on the cross and continues to be offered in the celebration of the Eucharist for the glory of God and the salvation of humanity. Incorporated in Jesus Christ, the baptized are united to him and to his sacrifice in the offering they make of themselves and their daily activities (cf. Rom 12:1, 2). Speaking of the lay faithful the Council says: "For their work, prayers and apostolic endeavours, their ordinary married and family life, their daily labour, their mental and physical relaxation, if carried out in the Spirit, and even the hardships of life if patiently borne-all of these become spiritual sacrifices acceptable to God through Jesus Christ (cf. 1 Pt 2:5). During the celebration of the Eucharist these sacrifices are most lovingly offered to the Father along with the Lord's body. Thus as worshipers whose every deed is holy, the lay faithful consecrate the world itself to God".
• Precisely with this in mind the Synod Fathers said: "The secular character of the lay faithful is not therefore to be defined only in a sociological sense, but most especially in a theological sense. The term secular must be understood in light of the act of God the creator and redeemer, who has handed over the world to women and men, so that they may participate in the work of creation, free creation from the influence of sin and sanctify themselves in marriage or the celibate life, in a family, in a profession and in the various activities of society".
• Even Christian married couples, in imitation of Aquila and Priscilla (cf. Acts 18; Rom 16:3 ff), are offering a comforting testimony of impassioned love for Christ and the Church through their valuable presence in mission lands.
• The lay faithful's duty to society primarily begins in marriage and in the family. This duty can only be fulfilled adequately with the conviction of the unique and irreplaceable value that the family has in the development of society and the Church herself.
• We can conclude by reading a beautiful passage taken from Saint Francis de Sales, who promoted lay spirituality so well. In speaking of "devotion", that is, Christian perfection or "life according to the Spirit", he presents in a simple yet insightful way the vocation of all Christians to holiness while emphasizing the specific form with which individual Christians fulfill it: "In creation God commanded the plants to bring forth their fruits, each one after its kind. So does he command all Christians, who are the living plants of his Church, to bring forth the fruits of devotion, each according to his character and vocation. Devotion must be exercised in different ways by the gentleman, the workman, the servant, the prince, the widow, the maid and the married woman. Not only this, but the practice of devotion must also be adapted to the strength, the employment, and the duties of each one in particular ... It is an error, or rather a heresy, to try to banish the devout life from the regiment of soldiers, the shop of the mechanic, the court of princes, or the home of married folk. It is true, Philothea, that a purely contemplative, monastic and religious devotion cannot be exercised in such ways of life. But besides these three kinds of devotion, there are several others adapted to bring to perfection those who live in the secular state".

My summary from this would be that first and foremost, the married laity should strive to live up to the exhortation of the saintly pope with the support of their pastors and fellow Catholics. Their saintly marriage will then be a testimony in season and out of season.

Other Catholic Institutions for example Opus Dei [their founder Saint Josemaría is referred to as the saint of ordinary life] assist their members, majority who are married, and any who avail themselves to their apostolate, to live a life of work, family life, and other ordinary activities as occasions for spiritual union with Jesus Christ.

I add this section because I can't help but think that your question has been prompted in some way by what is happening in the world e.g. in Ireland and in the US with their recent legal recognition to unions between h*m*sexual persons.

As to what a Catholic should do where legal recognition has been granted to unions between h*m*sexual persons, the pertinent document is the Congregation for the Doctrine of the Faith's (CDF's) Regarding Proposals To Give Legal Recognition To Unions Between Homosexual Persons whose paragraph 10 states:

10. [...] all Catholics are obliged to oppose the legal recognition of h*m*sexual unions, Catholic politicians are obliged to do so in a particular way, in keeping with their responsibility as politicians. Faced with legislative proposals in favour of h*m*sexual unions, Catholic politicians are to take account of the following ethical indications.

When legislation in favour of the recognition of h*m*sexual unions is proposed for the first time in a legislative assembly, the Catholic law-maker has a moral duty to express his opposition clearly and publicly and to vote against it. To vote in favour of a law so harmful to the common good is gravely immoral.

When legislation in favour of the recognition of h*m*sexual unions is already in force, the Catholic politician must oppose it in the ways that are possible for him and make his opposition known; it is his duty to witness to the truth. If it is not possible to repeal such a law completely, the Catholic politician, recalling the indications contained in the Encyclical Letter Evangelium vitae, “could licitly support proposals aimed at limiting the harm done by such a law and at lessening its negative consequences at the level of general opinion and public morality”, on condition that his “absolute personal opposition” to such laws was clear and well known and that the danger of scandal was avoided. This does not mean that a more restrictive law in this area could be considered just or even acceptable; rather, it is a question of the legitimate and dutiful attempt to obtain at least the partial repeal of an unjust law when its total abrogation is not possible at the moment.

I do not believe it is by coincidence that this document was issued June 3, 2003, Memorial of Saint Charles Lwanga and his Companions, Martyrs.

Cf. Bishop Strickland’s Statement on U.S. Supreme Court Decision

"Let me unambiguously state at the outset that this extremely unfortunate decision by our government is unjust and immoral, and it is our duty to clearly and emphatically oppose it. In spite of the decision by the Supreme Court, there are absolutely no grounds for considering unions between two persons of the same sex to be in any way similar to God’s plan for marriage and the family. Regardless of this decision, what God has revealed and what the Church therefore holds to be true about marriage has not changed and is unchangeable."

The Magisterium has offered a number of guidelines for how to live the institution of marriage, so as to make it truly life-giving, and indeed a source holiness for married couples. The best “defence” of marriage is the witness of couples who have lived out their vocation in accord with God’s plan for them.

The universal call to holiness

The Church’s recent Magisterium (since Vatican II) has emphasized that all people are called to holiness, not only those who have been called a clerical or consecrated vocation:

Thus it is evident to everyone, that all the faithful of Christ of whatever rank or status, are called to the fullness of the Christian life and to the perfection of charity; by this holiness as such a more human manner of living is promoted in this earthly society (Lumen Gentium 40).

Married couples are no exception: the Sacrament of Matrimony is their particular path to holiness:

Furthermore, married couples and Christian parents should follow their own proper path (to holiness) by faithful love. They should sustain one another in grace throughout the entire length of their lives. They should embue their offspring, lovingly welcomed as God’s gift, with Christian doctrine and the evangelical virtues (Lumen Gentium 41)

In other words, the grace received in matrimony is the source of their holiness, and it is fundamentally that grace, and the holiness produced by that grace—not something particular that the couples need to do—that will provide the witness that the Church asks.

The fundamentals in holiness of life

The principle “task” of married couples, therefore, is to live holy lives, and in order to do that, couples must first of all place no obstacle to living holy lives: they must, in other words, obey what the natural law tells them is necessary for a happy marriage.

First of all, the fundamental characteristics of marriage must be respected: marriage is by nature indissoluble (it cannot be ended at the discretion of the spouses), exclusive (it can only be with one spouse at a time), faithful (one cannot permit extramarital affairs), and fruitful (children are always to be welcomed). (See Familiaris Consortio, Part 3, chapter I.)

It must be specified nowadays that true marriage can only be contracted between one man and one woman, always considering the sex that they have “originally” had since birth.

The controversy over who is elegible for marriage is only one of the challenges to marriage today. There is also the relative ease with which couples can obtain civil divorce—which can easily lead couples to think that marriage is inherently transitory, not permanent—and the strong temptation to use contraception as a means to limit childbirth.

(We should note that the Catholic Church does not prohibit limiting childbirth for legitimate reasons; she merely says that actions that destroy or forcibly impede our bodies’ ability to procreate—e.g., using hormonal contraceptives or condoms, and obtaining tubal ligations or vasectomies—is immoral, because it tends to undermine both the unity of the spouses and, evidently, their God-given call to bring forth children. See Humanae Vitae, especially numbers 14-15. It is also worth noting that modern gynecology provides excellent and highly reliable alternatives to oral contraceptives, when regulating birth is necessary. See, for example, http://www.creightonmodel.com/.)

It is also important for couples to realize that they are making a very serious and permanent commitment to one another. Even if something should go wrong—either through illness, or even when one spouse is not faithful—the couple have an obligation to regard themselves as married. Separation and civil divorce can be tolerated when the situation warrants it (and when there is grave danger to one of the spouses or to the children, it might even be obligatory); however, “remarriage” is never a legitimate option, unless there is a way to prove in the external forum that the first marriage was, in fact, not a marriage at all. (See Familiaris Consortio 83.)

The first and most fundamental step, therefore, to being faithful witnesses to married love is to follow the Commandments and the natural law regarding matrimony. Nowadays, simply being faithful to the basics of morality constitutes a heroic witness, and for this a married couple can count on the grace that they receive in the Sacrament of Matrimony.

How to grow in holiness

Living a happly and faithful marriage, however, does not reduce to fulfilling the minimum requirements.

Part Three of Familiaris Consortio gives a number of suggestions in which couples can grow in holiness. Among others,

1. Married couples participate in a unique way in the creative power of God. The Church encourages couples to take part generously in the gift that God has given them to bring new human beings into the world. It is a common misconception that the Church wants couples to have as many children as physically possible. That is not the case: rather, they should take into account their means and abilities. However, the Church does encourage couples—without obliging them—to be generous, not miserly, since such generosity will bring about happiness and fulfillment. (See Familiaris Consortio 28.)

2. Married couples have an invaluable role in educating their children. In fact, parents (not the state) are the primary educators of their children, and the institutions that exist for that purpose are meant to be at the service of parents. (Schools and governments that usurp parents’ rights to educate their children, especially as regards sensitive matters such as sexuality, commit a most grave injustice.) It is, moreover, parents who first bring their children in contact with the Church and the Sacraments. (See Familiaris Consortio 36-40.)

3. Strong, stable families are the bedrock of society. Espcially since there is so much upheaval today, Catholic couples are called to be examples of fidelity, and thus offset some of the ills that plague society today. (See Familiaris Consortio 44.)

4. The family is an image of the Church (See Eph. 5:32), and moreover, the Church’s efforts to evangelize begin with the family. (See Familiaris Consortio 50 and 52.) For example, family members can help each other to come to the Faith, or to a fuller practice of the Faith, and some familes may even feel called to dedicate themselves full time to missionary apostolate, although this will not, of course, be the norm for most families. (See Familiaris Consortio 50 and 54.)

5. The Sacrament of Matrimony finds its fulfillment in prayer and the Sacraments, particularly in the Eucharist. Families draw their strength from prayer and participation in the liturgy. (See Familiaris Consortio 56-62.)

TL;DR

All Catholics are called to live holy lives, and married persons will find the source of that holiness in the Sacrament of Matrimony. In order not to place an obstacle to that holiness, at the least they need to obey what the natural law tells us about marriage: in particular, that it is the union of one man and one woman, that it is indissoluble and exclusive, that it entails fidelity, and that it must be open to procreation. This is, as it were, the minimum requirement for a happy and fulfilling marriage. Just living marriage faithfully is a powerful and even heroic witness. This, together with actively participating in the life of the Church as a family, constitutes the best “defence” of marriage that Catholics can offer.