According to the Catholic Church is it sinful to address someone who is transgender by their biological sex? As a Catholic, is it considered an offense against charity to refer to a person in a way they wouldn't want to be referred to (such as calling a trans woman "he")?
Detraction &amp; Railing
In other words, is it

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

or

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

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

St. Paul called the Galatians &quot;senseless&quot; (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 &quot;foolish,&quot; and the Apostle called the Galatians &quot;senseless.&quot; Yet, as Augustine says (De Serm. Dom. in Monte ii, 19), &quot;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.&quot;

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. &quot;Whether fraternal correction is an act of charity?&quot; (II-II q. 33 a. 1).