"men can't become women" is a common refrain I see TERFs use.
not to be a reddit debate nerd, but this is a really common informal fallacy that can be refuted pretty easily.
it's known as "begging the question", or "assuming the conclusion". it occurs when an argument's premises assume that the conclusion is true.
the argument extrapolates thus: "men cannot become women, therefore, trans women are not women, therefore, trans women are men".
or, to phrase the argument another way, "trans women are men, because men cannot become women, therefore trans women are not women, trans women are men."
notice that nothing in the argument supports the conclusion that trans women are men, but the premise that men cannot become women, which is supported by the premise that trans women are men.
we can debate whether men can "become" women (is "man" a permanent, immutable state? are trans women ever really "men"?) but those arguments are not even worth having in this context, because the conclusion being arrived at is faulty and argued in bad faith.
it is on the speaker to define precisely what a "man" or "woman" is, and then to derive from those definitions the impossibility of a "man becoming a woman".
when a person argues in bad faith, as in this case, you are not required to argue against it. in fact, you will tie yourself in knots trying to argue against it, because the well has already been poisoned before you even engage with it.
if you want to be polite about it, point out that the argument makes no sense and ask them to come up with a better one. if you don't, and you probably shouldn't, you can just tell them to fuck off.
as a holder of a philosophy degree, you have my permission.
