No problem! One of the reasons I became interested in primatology was because studying our closest living relatives can tell us a lot about our own behaviour.

If we are going to take the sexual dimorphism rule that seems to apply throughout the mammalian world, it would appear as though we have become more sexually monomorphic and less dimorphic over the past few hundred thousand years. Many palaeoanthropologists believe this is because monogamy has been increasingly selected for. From a purely functional perspective it would make sense that monogamy would be incredible beneficial for a species that has such a prolonged maturation period. For example, it takes chimpanzees and bonobos 6-7 years to become independent and sexually active. It takes humans considerably longer - in between 13-20 depending on culture and social system and a range of other important variables. It takes us so long to mature because our brains are so much larger (3 times larger than chimpanzee and bonobo brains).

Anyway, as we evolved, and our brains started to expand (in between 2 million and 100,000 years ago) we would have needed more and more bi-parental care (mother and father). It is during this same time that our human ancestors (e.g., Homo habilis, Homo erectus, etc.) became less and less dimorphic (i.e., the male and female sex ratio reduced considerably). Obviously we are not completely monogamous, but our sexual patterns more closely resemble that of gibbons than chimpanzees. We are still more promiscuous overall than gibbons but if our social-sexual system resembled the chimpanzees there would be no such thing as an exclusive long-term relationship (i.e. marriage).