I just read that it's more social monogamy than sexual monogamy in other species. There are different theories as to why a few species are that way. One is the need for two parents to look after the baby, since the babies in that species need more help. (The article did suggest that humans would fit into this category, by the way.)

Ha--interesting finding: There is a correlation between the size of the testes in a species and whether the species is likely to be more active in seeking out different mates. Note I said species! I didn't say in any given male!