Ignorance, insecurity, personal experience, and stereotypes that just because someone is more attractive and is more likely to have more options that they will just leave them for another guy. Which could be part of their own insecurity. A persons physical attractiveness doesn't equate to the greater chances that they will cheat and be unfaithful. All it means is that they have more options. It's the same thing with the girls that think that guys that are really hot are either douche-bags, a**holes, and/or unfaithful. Does that mean a guy that is hot is any of those? No. Just like a girl that is hot isn't more likely to cheat, isn't more likely to be a bitch, or anything than any other girl that is less attractive.