I’d describe it by explaining that it’s unhealthy to have a partner that doesn’t like who you want to be. A pre-op trans man is a man. Treating them like a woman during sex would likely make them feel dysphoric. Having a partner that they know wouldn’t be attracted to them if they actually transitioned would just suck. If you identify as a straight man, it would be invalidating of the trans man’s identity to fuck them anyways because you see them as a hot woman.
On the “chubby chaser” bit, think of it like this. I love fat girls, tummy soft 🥺 however, my girlfriend could lose weight if she wanted to. If she would feel more confident with 100 less pounds, that’s okay because it’s her body and I want her to be happy with her own appearance. If I got with a fat girl who wanted to lose weight and I made her feel bad about it because “then you won’t be hot anymore” that would be kinda shitty right?