General

What in the incel?

This is true. However, I fear they may be incompatible. He seems to put too much value on her appearance, and she seems to be both insecure and perfectly fine with cheating.

There's nothing wrong with not wanting to be bigger. She doesn't want to gain weight, and that is perfectly fine. It's one thing if all OP wanted was to feed her sometimes, but it seems he also wants her to change her body for his pleasure as well.

On the flip side of this, it's not cool for her to jump to the whole "I'll cheat on you" response. I get that she doesn't want to indulge his fetish, but the way she responded shows that she isn't trustworthy either. They might have found a compromise for his fetish, but there isn't a way to compromise trust.

I don't know what OP said to her or how OP said it, but it is clear as day that she has no interest in indulging this kink. OP would be wise in learning from this situation. You can date non-kinksters, but there's a way to do it. And this whole thing has been a train wreck.

OP, I cannot overstate the importance of talking to a therapist about your situation. From this small window you've given us into your inner world, I can see deep-rooted insecurities, shame, and maladaptive traits that will not serve you in navigating love and kink. Until you sort that shit out, you'll get nothing but repeats of this situation.

Well, there is something wrong with him wanting it to work out, she is harmful to his mental health. And that's not going to change.

Yeah, sometimes you can work shit out. Sometimes you can't. It depends on how willing both parties are. There's fault on both sides. Walking away is an option, but so is working it out.

This can be looked at on several levels. I think counseling with BOTH present is the only thing that might work. What I am seeing is a girl who now feels suddenly much more attractive and love the attention she is getting from other guys. The fact her boyfriend is "disgusting" for liking her fatter is an excuse for her to "move on". If she really does not understand his preferences and still loves him she will make an attempt to understand him better. Perhaps she is afraid that this is a "control" issue that he wants her fatter simply because he does not want any other guys looking at her. That happens as well. She has to understand that this fetish is very unlikely to go away and they will both have to compromise on some level. If you truley love someone then that is possible.