I question if monogamy is natural given that there is so much cheating going on. It seems we have to work hard to maintain it. There is also plenty of non-monogamous behavior going on in the animal world. Here's an article that brings up this point:
Institute for Family Studies
For Debate:
Is monogamy natural?
If it isn't natural, then why should we mandate it when it comes to marriage?
Institute for Family Studies
Among mammals, just 9 percent of species are monogamous; among primates, just 29 percent are. Humans are a diverse lot, but before Western imperialism, 83 percent of indigenous societies were polygynous, 16 percent monogamous, and 1 percent polyandrous (where women have multiple husbands).
What’s interesting, and frustrating for those looking for clean answers, is that these categories aren’t as exclusive as they seem. Polygynous human societies are only preferentially so: Many men still acquire only one wife, and Barash writes that such pairings were probably the most common throughout human history. Monogamous societies, of course, always have their share of philandering and “serial monogamy,” too. And there may have been a significant shift from polygyny toward monogamy beginning about five thousand to ten thousand years ago, judging by genetic evidence. Some kings and warlords aside, ours is a case of moderate polygyny; we’re not elephant seals, whose harems often number up to 40.
For Debate:
Is monogamy natural?
If it isn't natural, then why should we mandate it when it comes to marriage?