I mean, I was being facetious but let's unpick this.
Some fish can change sex. (Humans can't, but let's talk about fish.)
Whilst that process is under way, but before it is complete, other fish start to behave as though the fish is the sex it is in the process of becoming.
In what way?
Do the female fish start inviting the male fish who is in the process of becoming female on hen dos and shopping trips?
Do the male fish start inviting the female fish àwho is in the process of becoming male to play golf and offering her a beer rather than a prosecco at fishy BBQs?
Do fish tradesmen suddenly start manfishsplaining to the male fish who is in the process of becoming female and taking the female fish who is in the process of becoming male more seriously?
Or is it, as I suspect (because fish do not do any of these things) that fish start treating the fish which is in the process of becoming the opposite sex as a potential mate with whom they could reproduce?
If so, that is about sex, not gender or gender identity.
I, on the other hand, completely regardless of my sexuality, would not start sizing up a woman who was in the process of transitioning to a trans man as a potential father of my children. For fairly obvious reasons.
And even post transition I would not treat a trans person as their acquired gender in any real sense, other than perhaps using their preferred pronouns. Because I'm not afraid a trans man is going to rape me, but a trans woman might for the same reason that a man might.