he has historically been avoidant with these conversations

I would call that a clue that he might not be receptive to suddenly finding himself the partner of someone undertaking a make-believe sex switch. Plenty of people find GNC women attractive but would not find a FTM attractive. She owes it to him to tell him, but she should be preparing to let him off the hook and out of the relationship simultaneously, not ignoring that it's highly probable that he won't enjoy this betrayal, which is what it is likely to feel like from his perspective.

So many partners are gaslighting themselves in this regard. You didn't partner with a man. You aren't suddenly obliged to decide (!) you are gay because your female partner finally admits the extent of her mental health challenges—you just don't have a stable, reliable partner. Big difference.

And this whole situation with what they do to their partners is probably part of the fuel for claiming that being gay is a choice. Because FOR THEM, when they F over their partners with these surprises, they NEED their partners to make that "choice" in order to stay in the relationship. Furthermore, they need their partners to suddenly forget what biological sex is. So it's still not actually being gay (or bisexual), just saying you are, which is a lie in these contexts. So now, their partners have to lie to themselves and everyone else.

Other people's lives are not simulations where one can run live scenarios and decide if one likes the outcomes or not.