@AverageGuy
From what little I've read of evolutionary theory, it's clearly very imprecise. I have a book on my shelves called "Why is Sex Fun" by Jared Diamond. He makes all manner of interesting observations on why human anatomy is as it is, but he freely admits that it's all speculative, so I would say that what men or women are "designed" to do doesn't really get us very far.
But more importantly, it's all absolutely beside the point. If we take any species at all, and judge what is normal for that species, we simply observe what its members do. And for humans, that's monogamy with some exceptions. For humans, sex plays a role not just in procreation but pair bonding. It's ironic that you decry the role of religion (also a natural phenomenon, from the honest atheist's point of view) but identify the main purpose of sex as procreation. That's the traditional Catholic view and one not generally shared by other forms of Christianity or other religions.
It's not unusual for males and females of other species to copulate with multiple partners - why should we be any different? Because society tells us we shouldn't. It's "bad", "forbidden", "wrong", "dirty" and other such terms - particularly for women
Yes, but why does society traditionally tell us this? There's nothing new about people questioning this view, and no shortage of discussion on it. There are all sorts of reasons I might cite, but one is that a feature of humans is that we fall in love, and that tends to involve exclusivity. We also love our own children more than other people's children, and that creates an obvious impetus towards fidelity.
IMHO, Religion (generally written by men, you'll notice), has forced monogamy upon us as a race.
I apologise for being abrasive, but this is nonsense. If elephants exhibited group conduct that looked like some sort of religious service, no one would be saying that was unnatural for elephants. The idea that humans in a state of nature get to do what they like, like the Polynesians (before Europeans introduced a devastating clap epidemic) ignores the fact that humans are social and always have their rules about such things. Whether or not those rules are based on belief in God, gods or some other kind of divine force is irrelevant.
All this comes from the misconception that because plenty of monogamous relationships fail, humans aren't designed for relationships. It would be more accurate to say that humans, especially if offspring are involved, are most likely to be in stable monogamous relationships that will last a very long time unless something goes wrong.
I've never seen any good evidence that things like hotwifing and cuckoldry are statistically important so I don't think that disproves my point. To be honest, I think people like the idea of other people getting up to that sort of thing because it's daring and in theory exciting. And I think that can lead people to think that it's normal, when in fact it is (as a matter of fact, not as moral judgment) not normal.