As I understand it, men are actually naturally polygynous, while women are serially monogamous. However, what we call God’s plan for sexuality is obviously less cruel than us simply succumbing to our own genes.
I know that's the conventional wisdom, but I'm not sure I believe that. Most (too lazy to look up, I think around 75%) men never cheat on their wives. I don't believe it's because they are all such devout church-goers.