It has everything to do with selectivity and finding the best relationship available. Everyone is trying to get the best partner they can. I'm not saying fat, poor, people don't get married. The reasons rich, powerful, beautiful people intermarry is because they can. They have the resources to justify being with someone on their level. The reason overweight poor people intermarry is because they have to because they can't do any better. The overweight, uneducated welfare bum isn't married to Giselle Bundchen not because he doesn't like her, it's because he doesn't have a chance in hell with her. Your theory might have some credibility if you showed significant examples or stats of people with resources like wealth, beauty, and power marrying people who were poor, ugly, and weak. But it doesn't happen, that's the point. People generally intermarry with the same socioeconomic status and looks. So if you have more or better resources like money or looks ("more desirable traits"), the more likely you are to end up with someone else with more or better resources. Show me significant examples otherwise and I'll eat my words.
see the review