Most Helpful Guy
A lot of guys think pretty girls are more likely to cheat because they have more options, with better quality partners. So pretty girls have to deal with more temptation. Pretty girls can also replace their partner so easily that they often don't value their partner as much as she should.
I actually think ugly girls are the most likely to cheat, because they feel like they have something to prove. Girls that are somewhere in the middle are actually seem to be the least likely to cheat in my experience. Of course it really does depend on the individual.