I would consider "natural" to mean our basic inclination at a more primal level - and to determine what our natural inclination is, I would look at how we humans have behaved in the past and currently behave.
People, for the most part, date in highschool and form relationships, then break up, then form new ones, then break up. This continues until marriage, where a lot of people still cheat on their spouse and where a large portion end in divorce. From that, I would gather that we would naturally fall in line under serial monogamy. I would count pairings lasting for a long time (until death) as being one particularly long monogamy in a series, and would be aided by social assumptions (monogamy is reinforced by society's judgment) and by that particular person's "style". There are differences everywhere though, so I would consider it quite variable but still falling in line mostly with one style.
As far as cheating goes, I'd be inclined to say that it would be due to trying to replace our natural urges with ones held up by society. Those aspirations would most likely fail in practice, but would still be beneficial to keep up in appearance. However... on a more biological note...
I would attribute major causes of females cheating to wanting to have their cake and eat it too. By that I mean being socially monogamous with the provider type (rich, but kinda lame) for security, while wanting to mate on the side with the 'alpha' guy for genetic inclinations (subconsciously).
For males... I remember reading about how when a couple has a child, then man's testosterone levels drop - that may be a reason that men are less likely to cheat on their spouse once kids are involved if that is, in fact, a true statistic. 'Alpha' type guys would appear to have more testosterone than others, and are more inclined to use force and take what they want - which would include additional partners if they so desired... and many would. However, the drop in testosterone may be linked with a contentment with a single partner and less of a desire to stray from the marriage.
My thoughts on the subject, anyway.
On a side note, I'm particularly interested to see what happens with my dear sister concerning that above point on testosterone reduction. She married a guy who I consider a complete douchebag and just had a kid with him. I'm curious to see if he softens and becomes more of a "nice guy" as the new reality settles in.