Since the ratio of males to females is about 1:1, and the raising of young takes so many years, it would imply humans are meant to be monogamous. Marriage is really for raising a family, a woman will know her children if she's monogamous or not but the only way a man could hope to know his own children was to convince a woman to live in a monogamous relationship.
That said I do believe that we are serially monogamous .. mostly... that means that while we are with a partner we are monogamous but then the desire is to move on to another monogamous relationship.