Here follows a hypothetical scenario, which may or may not resemble a conversation from real life.
Alfa: Man it would be sweet if Sam Smith dated G-Flip.
Beto: You think so? I think she’s too hot for him these days.
Alfa: Whoa there, Beto! Pronouns, please!
Beto: LOL, wut.
Alfa: You have to use they/them for Sam & G, because they don’t see themselves as either men or women.
Beto: I do see them as a man and woman, though. In fact, I know which one would be at risk of being impregnated by the other one if they didn’t take prophylactic measures.
Alfa: I meant “man” and “woman” as gender identities, not as signifiers of sex.
Beto: Okay, but you could tell what I meant by “man” and “woman” though.
Alfa: Fine, but you have to use the pronouns that they would have you use.
Beto: Why? They aren’t even here.
I'm not going to answer Beto's question here, because that would put me at risk of strawpersoning the argument in favor of using pronouns to signify gender identity rather than sexual physiology. Suffice to say that Beto is listening with an open mind, and might well be persuadable.
p.s. Dunno whether to post this in the main trans thread or here in the pronoun thread. I’ll try it here first and if it gets bitbucketed then I’ll repost over there.