Some people like being fat. Some people like being women.

A person who likes bobs and vagene entering a committed relationship with a trans man would be toxic. Ditto for a person who likes fat people entering a committed relationship with someone who is trying to lose weight. That said, firstly, I don’t see anything wrong with a one-night stand in either of those scenarios, which is what the original question was, and secondly, more importantly, you’d have to stretch really hard to say that a person who’s into a little chub and a person who’s perfectly happy having a little chub entering a committed relationship is in any way problematic. OP is into women. If anyone unironically tries to call someone a pussy-chaser I’m going to recommend them for commitment.

OP saying he’d have sex with a trans man who hadn’t transitioned yet is no different than a lover of booty saying they’d have sex with someone who hadn’t lost weight yet. Sure, it might be a little disheartening for his potential sexual partner, but if it’s only for one night, and that person otherwise has a good support network, and, most importantly, it’s two consenting adults, I don’t see the harm.