Some of them come with fun extras like dual air bags. It means that we like you enough to sleep with you, but not enough so that we want to see you repeatedly. Generally, it means that the recipient of said comment is physically repulsive enough that no beer goggles may be thick enough to provide adequate protection. It is genetically inherited behavior. It's been passed on from our most primal forefathers, and it'll be passed on to our sons.
Several factors are at work, namely evolution, heredity, nutrition, and environment.
It is like with all things. Face it, if men were obsessed with ugly women, there would be just as much bitching about why men are so obsessed with ugly women. No matter how you set this up, some people are always going to be left out. I don't see anyone screaming about equal treatment for the stupid people either. Well, let's see.