Who says monogamy is the norm. Over 50% (sometimes as high as 60%) of all marriage results in divorce, and almost all humans have multiple sex partners, especially the males. Our genetic make-up actually supports this. The way male sperm works actually makes it more benefitially for males to have sex with as many partners as possible because the rate of insemenation is so remote that by sticking with one mate you actually decrease your chances of producing viable offspring. Moreover, you move down the food chain to our next closest relative; the Chimp and they do it all over the place, and are not monogamous. Group sex, oral sex, multiple males, multiple females, are all very common sex practices among chimpazees. Moreover marriage, in early human civilization only seemed to apply to lower classes. In almost all cultures those who are the best at competing for resources are rarely monogamous.