Peruano, I have a positive opinion about open relationships in regards to satisfying us emotionally and sexually.
People still might want some strong kind of partnership to raise a family I believe, but it need not be marriage.
To me the only reasons these kinds of relationships haven't been accepted is because people strongly believe in what society showed them a relationship should look like, they have FEAR of what others will THINK of them if they themselves were in such a relationship, and they have an intense jealousy that exhibits itself in several ways, mostly through not wanting to see others enjoy the same kind of relationship with someone they like, but also seeing third parties do this.
As for why people cheat, I can only guess it is because they want to be with someone different. Variety is a good thing in my opinion in nearly all facets of life. I hate to use a food analogy, but it works. If you try a another style of your favorite dish, even if it turns out to not be as good, you will still enjoy it, and chances are you will enjoy your favorite dish even more the next time you have it. When we don't allow ourselves or our partners to be open about who we befriend or get intimate with, and make ourselves afraid of being honest about how we feel, why then do we expect our relationships to not become dishonest?
There are more complex reasons for specific examples but I think that the want of a variety of emotional and sexual relationships is the main reason.