I don't think you'll get much of a response to this. The trouble with MN is that people have cultivated OL identities on here and it's still a community of sorts. While it's safer than discussing it IRL and people can of course name change, I think many posters will still be reluctant to admit to it on here.
I'm not at all convinced that life-long monogamy is the natural state for humanity, but even while accepting it is a cultural invention, I still condemn cheating. It is up to every individual to decide whether or not they want to be monogamous. If you agree to be in a monogamous relationship you should be honour your promise or if you find yourself in a situation where you feel you can no longer do that, you should break your 'contract' in as kind and respectful a manner as possible (i.e. not lying, manipulating and leaving someone else high and dry).
All that said, I think most people don't really think about it until it's a bit too late. Most people have affairs because they haven't protected themselves in the event of having the opportunity to have one. Few people go out seeking affairs IMO.
The reason men seem to have had more affairs is IMO due to having more opportunity rather than anything linked to the Y chromosome. Culturally, up until very recently women in relationships would find themselves much more in the company of other women rather than men and often with children in tow.
Also, fear of loss is a great motivator. For women, who are often the lower earners (if earning at all), the loss of financial security has a way of concentrating the mind on the vows of monogamy much more, or certainly to the extent of being more discreet if cheating. For men, until very recently they had nothing to lose even if caught. The law enshrined their rights over those of their wives even if the wife left because of her husband's cheating. That's now changed, but the effects are longer lasting.
I don't think either sex is more likely than the other to cheat. I think gender roles have a big influence, but that's quite different.