I can sort of relate to this situation. My girlfriend steadily put on a lot of weight about a year and a half after we started going out. I noticed, but I didn't mind a bit of weight to begin with. After a while she was really quite overweight, but I love her to bits so I just can't bring myself to break up with her.
She knows it bothers me, but she doesn't seem to have the will power to make a change to her life. I still want to be with her, but I just can't help looking at hot girls and thinking I wish my girlfriend looked like that. Don't get me wrong, she is really pretty, but I just don't find her body attractive.
You can say that I'm a bad boyfriend for feeling this way, but at the end of the day, you don't know how hard it is to stay with someone who you are not physically attracted to. Sure, I might be able to put up with this, because she means a lot to me. But I wouldn't judge somebody else for leaving their partner for this reason, nor would I blame them.
I bet the people who are criticising the OP have never been in this situation, maybe you would understand once you were