Honestly, it's nature. People are attracted to health, period. It's the number one factor in sexual attraction. This has NOTHING to do with men being shallow or anything like that.
I'm not a therapist, but I'd say you should work out your weight issues before your relationship issues, as being overweight is unhealthy and could harm you whereas no sex likely won't kill you as fast.
Oh, and you see overweight men with beautiful women because these men have money. Women don't have the same attraction to men as men do to women. In my experience, women look for someone that can take care of them financially and emotionally. Now, if he's fat and poor and a bad listener, he likely has a massive schlong! I'm sorry to be so frank, but it's the truth as I see it.