1.) A close but not immediate family member recently decided to gender transition, which forced me to form specific and concrete beliefs about transgenderism before I was confident of being able to do so.
2.) Transgenderism is new and so we get little direct help from the saints or holy doctors. Because of this I’m bothered that all responses to the question will have at least a large dollop of extremism. As always, the problem is that we don’t have enough saints.
3.) I don’t know how many things transgenderism is, and it’s difficult to find any sex-related topic that has a uniform description for both men and women, viz. we leave out a lot if we try to understand sexual liberation, consent, commitment, or divorce as affecting men and women uniformly or if we try to understand homosexuality abstracting from the difference between gays and lesbians.
4a.) General moral axiom #1. If you speak in contradiction to all known physical evidence about a physical fact you’re lying, and lying is always wrong. Whatever else sexuality is (and this is a lot) it is also a physical fact. If I say I’m a woman despite my masculine genitals, skeletal structure, fetal development, hand strength, amount of striated muscle, different life expectancy, different healthy BMI, my risk of testicular cancer, creation of small, fast gametes, the presence of a Y-chromosome in trillions of cells of my body… then I’m mistaken, and if I insist otherwise in speech I’m lying. If I demand you agree with me in speech even though you have the same evidence I do, I’m demanding that you lie.
4b.) I’m open to discussing the possibility of disparities between chromosomal, gonadal and brain-developmental sexuality, but we can set all these aside – and in the case of my family member they aren’t relevant- and still have a massive amount of transgender cases left on the table.
4c.) I’m open to some standard of “being male” or “being female” that can work in defiance of manifest physical evidence, but I have yet to see how it can work. There is no non-physical account of masculinity any more than there is a non-physical account of having a 48″ vertical jump or a cancerous tumor, i.e. if you’re convinced you have either of these in spite of never jumping four feet or having no tumor show up on a full-body scan, then your conviction is mistaken, and if you insist on it in speech it’s a lie.
4d.) A lie is speaking falsely with the intent to deceive, and one can object that even if transgendered person is mistaken he need not intend to deceive. No doubt the transgendered persons wants you not to be deceived about who they are. But we intend not just what we know but even what we ought to know, and if there is anything one ought to know it’s that there is no account of sexual identity that can contradict physical evidence. The lie remains.
At any rate, this is not relevant in my own case. If I am convinced there is no account of masculinity in contradiction of physical facts, then I would lie by agreeing with someone who denied this.
5.) Not all lies are mortal sins, but it’s hard to see how this sort of lie avoids being one:
If, however, the [lie] be about something the knowledge of which affects a man’s good, for instance, if it pertain to the perfection of science or to moral conduct, a lie of this description inflicts an injury on one’s neighbor, since it causes him to have a false opinion, wherefore it is contrary to charity, as regards the love of our neighbor, and consequently is a mortal sin.
ST II-II q. 110 a. 4
6.) Moral axiom #2: If your disapproval of my doing X puts me at greater risk of suicide or self-harm, it is not evidence your disapproval is wrong. Whether your approval is right or wrong is determined before I decide whether to harm myself in response. If in disapproving of X you disapprove of something good then your action is evil whether I harm myself or not; if approving of X means approving something evil then your action is evil no matter what my response is.
Though I can’t figure out how to put it as an axiom, there is something fishy about approving of someone’s actions to keep him from harming himself. To be sure, to bully and berate others is always wrong, but no reasonable account of bullying allows every case of disapproval to be an instance of it. At any rate, it is also wrong to demand others approve of evil, doubly so if you extort the approval them by threatening to harm what they love, even if this happens to be yourself.
7.) I don’t know how often #2 is relevant to transgenderism, but it’s relevant in my own case, and it is an argument I’ve seen pop up before among sexual minorities.