Basically, the biological evidence is completely out, largely because you can't read genetic tendancies from socialized behaviour (well, you can, it's just bad practice). Though there does seem to be good money in writing books arguing one way or the other.
The traditional argument has tended to be that men are naturally more inclined to roam, but that women have evolved to catch men in long term monogamous unions. It's been argued that this is why we don't have a clear fertility cycle with periods of 'heat' like many other mammals.
Recently though, this has been pretty widely critiqued, mostly by pointing out that the assumption of women being 'less sexual' can't really be separated from the fairly general cultural rule that women tend to suffer more negative consequences and/or prohibitions against being sexually agentive and promiscuous, and that the breakdown in traditional tabboos about female sexuality does lead to women having more sex.
Recently there's been a good bit of 'debunking' literature about the idea that humans are naturally monogamous. But really, noone knows anything. The best policy is probably to assume, that others have said, that that if there is an overarching genetic 'intention' in very complex and socialized human behaviours then it's impossible to read and best ignored.
Personally, I feel that the social expectation that you'll meet someone in your twenties and still feel the same way about them when you're 80 is a bit ridiculous, and that noone is honestly in a position to make that kind of commitment. But I don't think framing it around genetics is necessarily the right way to go. It's better to look at it in terms of social and personal happiness than any kind of overarching genetic determinism.
It is true though that an awful lot of people who would claim to be monogamous cheat on their partners. For me though that's primarily a question of what's wrong with monogamy as a social necessity rather than as a genetic predisposition or otherwise.
