I think that humans are wonderfully complex creatures and I think that there are many reasons why people cheat. You've answered most of them towards your original post. Unhappy, missing something, bored, addicted to the thrill, downright greedy coming to my mind.

Nature v nurture. That's very interesting, but I like to believe that we've evolved from the need to spread the seed with as many people as possible. I did say, I like to believe! For me, someone who goes through life with multiple partners says more about their personality, and perhaps why they are never able to settle, than the need to spread seed. Why is it some people can be perfectly happy with one partner, and another never find the one? I think if it was just nature, then everyone would want to stray.

Cheating I think is very destructive. Even with someone saying that they're ok for someone to have other partners tends (not always) to end up with one person hurt, or more. I think because we are more than just bed mates, we have many other emotions that come into play and thats why someone gets hurt.