That is a question that I'm sure that the evangelical Christians would answer "yes" to even if they liked to go dabble in extramarital affairs. Others may have a more open-minded view on the matter. As I am starting this topic, I'm doing a Google search of the question just to see what I might find. Interestingly enough, my personal opinion is no. People can be monogamous, but it takes a certain level of restraint. I'm not saying we're necessarily meant to be totally promiscuous, though many people are. But we're not beavers either. Considering that a lot of men and women cheat at one time or another, it's not likely that humans are meant to be monogamous, at least not entirely. According to this article, humans are mildly polygamous. That's not a surprising determination. I think the idea of monogamy is more or less a social construct rather than a natural occurrence for humans. I'm not one to say that it can't be done, because it can. However, people need to be more honest about what they want.
It's certainly never a good idea to "cheat". My idea of cheating is when you have sexual encounters with people other than your significant other rather he/she is your wife/husband, boyfriend/girlfriend, life partner, or whatever you call him/her without permission to do so. If it's done with consent, it's not cheating, and if that kind of arrangement is better for both people involved, then it is probably a good idea. If there isn't agreement in the area, then there is a potential for cheating. The problem is that many do not communicate about the issue, and cheating becomes an issue about 22% of the time. All-in-all, I thought it would be significantly more than that. It's mostly out of fear that people do not cheat. I have no idea what percentage of people are in open marriages/polyfidelitous relationships, but my guess is less than 10%. Is this because society says that we should or because it is natural for humans to be monogamous? I personally think it's social, but I'm sure someone will disagree.