When the man looks better than the woman in a relationship, neither party is really going to be happy. Men like to chase, and women like to be chased. When the man looks better than the woman, he isn't going to feel like going the extra mile to keep her happy. He'll still sleep with her because he's a man, but he will generally have a nonchalant attitude towards the relationship. The woman in this relationship will be unhappy because she will feel undesired and unattractive. She will also have constant fears about her more attractive partner cheating on her. Not to say that beautiful women don't have cheating fears, but it's probably more prevalent in women who are more insecure about their attractiveness.

When the woman looks better, both parties are happy. Men enjoy having to work to please their hot girlfriend/wife, while the woman feels desired and attractive.