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 confusing and/or disheartening for his potential sexual partner to be told that he likes them for the one aspect of themselves that they’re working to change, 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.