Q: Is it ok to break up with your partner because they put too much weight on during the relationship?
A: Of course it is; a romantic relationship should be based on attraction on all three levels: body, mind and soul. If someone becomes gross physically and it turns you off and the thought of having sex with them or kissing them disgusts you, it's only fair for both parties that you call it off.
There are lots of people who don't seem to care that much about appearance, but I think appearance is a part of who a person is and not caring about appearance is like having a relationship with someone without caring about their personality because, <sarcasm>hey, rejecting someone based on having an ugly personality would be shallow, everyone deserves love and it's what's on the inside that counts, don't we all spring from the same source after all?</sarcasm>
I don't understand how a person can throw aesthetics out of the formula for attraction, and it doesn't seem right... Even if it is right for some people, though, it's also perfectly acceptable to care what somebody looks like, that's what nature intended. It fulfills the evolutionary function of sexual selection. It promotes beauty in the human form. It ensures that there's no dissonance between your visual impression of someone and your emotional impression of them. And in fact, one's physical appearance is their first and primary form of presentation to an observer, so it's only fair that we associate the fairness of their looks with the fairness of their identity.
The idea that people are shallow for not being attracted to ugly girls is part of the new PC agenda. It's good to be respectful and tolerant of people, but the movement can go overboard. And yes, fat is ugly, despite the even more modern movement to normalize it which is clearly born of the fact that obesity and overweight-ity have become so widespread in our culture that there are almost more fat people than there are healthy people.
Edit:
One more thing: As I argue in my essay Yes, Dogs Smile, it's probable that one's facial characteristics actually correspond in a profound way to their personality characteristics. And, while it's obvious that being ugly doesn't mean you're a bad person and being pretty doesn't mean you're a good person, it's very possible that being ugly confers some sort of fundamental but subtle imbalance in one's being, and that therefore by being intimate with them, their undesirable imbalance may rub off on you.