Well, monogamy was designed to make sure a man takes care of the children he has with a woman. It's an old fashioned approach to say the least, but something deeply rooted in tradition. Nowadays, it's much easier for a woman to keep from becoming pregnant by a man and women have their own means of support, but still, monogamy is the social norm and thus has a great deal of pull in modern society.
Ethically, monogamy is a choice between two partners. When there are children in a relationship, I believe it's the best way to go, but even childless couples can suffer from jealousy. I am married myself and would not be able to handle my husband being with other women and then coming home to me. I have know people in "open" relationships, but seriously doubt that they work in the long run. People are so prone to feeling neglected or jealous when they aren't being paid attention to or another person is getting more attention(or is more attractive, makes more money, they fall in love with each other, etc.).
Monogamy is just easier. Would you really want to have several relationships with multiple women and the associated children? I can't even fathom the amount of money that someone would have to make in order to support such a situation!