Supposedly, men are dicks and they are promiscuous, they want to fuck as much women as they can, and women are monogamous, they want one faithful partner. Of course, that is just the ideology and the way things work has nothing to do with it.
As things go, the very opposite might make more sense. I know you think this is nonsense, but i’ve got a very good explanation!
My muse Olivia Judson (talk about hott) has a blog post about monogamy and what actually caught my eye was the idea of “social monogamy” — being monogamous for social purposes (e.g. raising children) but otherwise sexually promiscuous. She cites some examples of animals known to be socially monogamous and appends the list with “some humans”. But… really, are only some of the humans like that, or do we all still feel sexual pull towards other people when we are committed? I don’t know, i cannot but think that we should deal with the extra-conjugal relationships instead of prohibiting them. I think being social monogamous is cool. Even more if the couple do talk about it.