You're in a long term relationship, your partner is very attractive and has always been fit. At some point he/she becomes quite overweight, would you leave him/her because his or her shape changed massivly, or would you stick by your partner?
Nothing in your relationship has changed only the physical aspect.
As for how overweight I'm talking about, just go to the point where you feel It's no longer ok, I guess you probably know where that starts.