BoopieBop:
I mean if you love them you'll stay with them. Peak fetish is nothing compared to actually having a connection with your partner. Be happy for them!
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.