This depends upon the circumstances, and should follow the same guidelines one would have in administering a reprimand towards another, (Fraternal Correction). A person should consider reprimanding another for sin if

1) There is reason to assume that the reprimand will be well taken and will contribute to the good of the person in question.

OR

2) If your silence would be seen as consent to or support of the sinful action in question. This leads easily to scandal, meaning you influencing others around you to think that the sinful action in question is okay.

So, of these principles, I find the second to be more touching on this issue. By calling someone by their preferred pronouns, you are actively partaking in a falsehood, and this would certainly be seen as supporting or consenting to the delusion of the transgender person. In the vast majority of cases then, I would refrain from using their preferred pronouns.

However, there is a slim case in which I think it is permissible. If the person you are talking to and those around you know without a doubt your catholic position that you are NOT supporting or consenting to the falsehood of transgenderism, and you can be reasonably sure that everyone around would not be scandalized, then I don't see a problem with using preferred pronouns in order to be polite.

Sources:

Definition of Scandal, from the Summa Theologica (II-II:43:1)

I answer that, As Jerome observes the Greek skandalon may be rendered offense, downfall, or a stumbling against something. For when a body, while moving along a path, meets with an obstacle, it may happen to stumble against it, and be disposed to fall down: such an obstacle is a skandalon.

In like manner, while going along the spiritual way, a man may be disposed to a spiritual downfall by another's word or deed, in so far, to wit, as one man by his injunction, inducement or example, moves another to sin; and this is scandal properly so called.

Now nothing by its very nature disposes a man to spiritual downfall, except that which has some lack of rectitude, since what is perfectly right, secures man against a fall, instead of conducing to his downfall. Scandal is, therefore, fittingly defined as "something less rightly done or said, that occasions another's spiritual downfall."

The principles I stated for Fraternal Correction are simplified more or less from this Catholic Encyclopedia page

Fraternal correction is here taken to mean the admonishing of one's neighbor by a private individual with the purpose of reforming him or, if possible, preventing his sinful indulgence. This is clearly distinguishable from an official disciplining, whose mouthpiece is a judge or other like superior, whose object is the punishment of one found to be guilty, and whose motive is not so directly the individual advantage of the offender as the furtherance of the common good. That there is, upon occasion and with due regard to circumstances, an obligation to administer fraternal correction there can be no doubt. This is a conclusion not only deducible from the natural law binding us to love and to assist one another, but also explicitly contained in positive precept such as the inculcation of Christ: "If thy brother shall offend against thee, go, and rebuke him between thee and him alone. If he shall hear thee, thou shalt gain thy brother" (Matthew 18:15). Given a sufficiently grave condition of spiritual distress calling for succour in this way, this commandment may exact fulfilment under pain of mortal sin. This is reckoned to be so only when

• the delinquency to be corrected or prevented is a grievous one;
• there is no good reason to believe that the sinner will adequately provide for himself;
• there is a well-founded expectation that the admonition will be heeded;
• there is no one else just as well fitted for this work of Christian charity and likely to undertake it;
• there is no special trouble or disadvantage accruing to the reformer as a result of his zeal.

Practically, however, individuals without any official capacity are seldom impeachable as having seriously transgressed the law in this matter because it is but rarely one finds the coalition of circumstances just enumerated.

Of course the reproof is to be administered privately, i.e. directly to the delinquent and not in the presence of others. This is plainly the method appointed by Christ in the words just cited and only as a remedy for obduracy is any other contemplated by Him. Still there are occasions upon which one might lawfully proceed in a different way. For instance

• when the offense is a public one;
• when it makes for the prejudice of a third party or perhaps even the entire community;
• when it can only be condignly dealt with by the authority of a superior paternally exercised;
• when a public rebuke is necessary to preclude scandal: witness the withstanding of Peter by Paul mentioned in the Epistle to the Galatians (2:11-14);
• when the offender has already in advance relinquished whatever right he possessed to have his good name safeguarded, as is the custom in some religious bodies.

The obligation of fraternal correction, so far as private persons go, does not obtain, generally speaking, for the case of one who violates a law through invincible ignorance. The obvious reason is that there is then no immunity for it is their duty to instruct their subordinates. Every one, however, whether having an official competency or not, is bound to give the admonition when the sin, committed though it be from ignorance, is hurtful to the offender or a third party or is the occasion of scandal.

Detraction & Railing

In other words, is it

1. backbiting/detraction (detractio)
   _{"the blackening of another's good name by words uttered in secret"
   —St. Albert the Great, Sum. Theol. II 117, quoted in St. Thomas Aquinas, Summa Theologica II-II q. 73 a. 1 arg. 1}

or

2. railing/reviling (contumelia)
   _{the same, but done openly, to his face (ibid.)}

to call someone other than what he or she would wish to be called?

Examples where it is not sinful

Directly to their face and certainly against their will, Jesus Himself truthfully called the

1. "Pharisees and Sadducees" a "brood of vipers" (Mt. 3:7);
2. "scribes and Pharisees, hypocrites" and "whited sepulchres" (Mt. 23:27);
3. and even the disciples themselves "foolish and slow" (Lk. 24:25).

St. Paul called the Galatians "senseless" (Gal. 3:1) and the Romans (Rom. 1:29-31) as

Being filled with all iniquity, malice, fornication, avarice, wickedness: full of envy, murder, contention, deceit, malignity: whisperers, Detractors, hateful to God, contumelious, proud, haughty, inventors of evil things, disobedient to parents, Foolish, dissolute: without affection, without fidelity, without mercy.

St. Thomas Aquinas explains why these name-callings were not sinful (II-II q. 72 a. 2 ad 2):

Just as it is lawful to strike a person, or damnify someone in his belongings for the purpose of correction, so too, for the purpose of correction, may one say a mocking (conviciosum) word to a person whom one has to correct. It is thus that our Lord called the disciples "foolish," and the Apostle called the Galatians "senseless." Yet, as Augustine says (De Serm. Dom. in Monte ii, 19), "seldom and only when it is very necessary should we have recourse to invectives, and then so as to urge God's service, not our own."

Fraternal correction is an act of charity.

In the case of those who wish they were the other sex, it would not be a sin to correct them with words they'd consider offensive. Fraternal correction is an act of charity; cf. "Whether fraternal correction is an act of charity?" (II-II q. 33 a. 1).