So today I stumbled across this article.
For those who can't be assed clicking the link and reading it (ie most of you), it basically states that we humans are not biologically inclined towards monogamy. Monogamy, as this bloke puts it, is a learned behavior; we may not be naturally monogamous, but we can certainly choose to be.
He says that:
"...there can be no serious debate about whether monogamy is natural for human beings. It isn't. A Martian zoologist visiting planet Earth