What are some of the reasons for which mainstream non-Catholic Christians are against gender-neutral secular marriage?

The Christian Church cannot support h*m*sexuality based on numerous scriptures, perhaps the most common and direct coming from Romans chapter 1. On the other hand, Christians cannot expect the world to live by God’s rules if they do not recognize God. That is literally between them and God. More to your point, the concept of gender neutral marriage requires the “re-definition” of marriage. In other words, taking a word that has a specific meaning and changing that meaning. If I were to take the word mortgage and re-define it to suit my purpose, I can assure you the bank would be quick to protest and, rightfully so. Words have meaning; and if we arbitrarily assign new or revised meanings to words, then truth no longer exists because we can shape it as we see fit if enough people agree. This particular word is Christian or, at least, religious in nature and has become secularized over time. The significance of the now mandated acceptance of GNM is that Christians with sincerely held beliefs about the morality of h*m*sexuality and h*m*sexual marriage will be forced to choose whether to hold true to their beliefs or face the penalty of law. Under these circumstances, who now is being persecuted?

God has a plan for His children which involves helping us live happy lives and developing God-like attributes. The plan started with Adam and Eve in the Garden of Eden:

Genesis 1:27-28 27 So God created man in his own image, in the image of God created he him; male and female created he them. 28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth,

The commandment given to Adam and Eve to multiply and replenish the Earth applies to us today as well. A couple that does not consist of a man and a woman cannot fulfill this commandment.

Genesis 2:24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.

Here God gives us a commandment which goes along with the one about multiplying and replenishing the earth. Why would God give us these commandments if they weren't for our own well being?

Marriage between a man and a woman constitute the basic building blocks of society. It is in this framework that children are most likely to get the love and care that they need to be healthy individuals.

A healthy society is made up of healthy families.

One of the concepts in ethics is universalizability. Which is to ask what if everyone did it? If we all turned to h*m*sexuality, you can see that in a hundred years, the earth would be de-populated. Thus, God's purposes would be thwarted.

Family is important to God. Look at the promises made to Abraham in Genesis 22:

17 That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies; 18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.

Our culture values living for yourself without regard for what is good for society. With no fault divorce, if either partner in the marriage decides its too inconvenient to stay married, then a divorce is granted. This concept of marriage for convenience extends to incorporate the concept of same sex marriage. In both cases, the idea is to grant as much liberty to adults without regard to how it affects society at large, or children specifically.

The God of Christianity is a God of sacrifice. He sent his only begotten son to redeem his children. Just as Abraham was commanded to sacrifice Isaac, God wants us to give him a broken heart and contrite spirit. How can we learn to do that when our natural tendency is to live for ourselves? By entering into marriage and having children, we learn to sacrifice and gain a perspective on life that is tough to gain in any other way.

Christians believe that God's law is not an arbitrary set of rules designed to make life inconvenient, but that it represents God's intended pattern for living a happy, successful, fulfilled life.

Many Christians believe that following God's pattern for human behaviour therefore results in more fulfilled people and a better society, whether or not those people believe in the source of the pattern (God). Many also believe that it is the duty of secular authorities to promote moral behaviour; and especially the duty of Christians to use whatever influence they have on the government to make it promote moral behaviour.