woah woah woah, I feel like we are doing a lot of demonizing of the polyamorous here, and I have a hard time listening to it.
There are lots of people who make monogamous commitments and then go ahead and cheat. Some of those people might blame the cheating on their biological need to spread their seed. These people are jerks. They are not, however, the only people who believe that humans are not naturally monogamous. There are plenty of honest ways to sleep with more than one person in your entire life. You find partners with similar beliefs, you hammer out an agreement that works for both of you, and then you are open and honest about what goes on. You could be sleeping with other people or merely admitting that you fantasize about it, but either way, you are discussing your non-monogamous desires.
If it were natural to be monogamous, we wouldn't fantasize about non-monogamy. Whether we act on our urges to be non-monogamous is up to us, sure, and it's up to every individual to do the right thing, but our physical bodies feel the urge to sleep around, even if our human sensibilities are able to suppress those urges.
If we weren't naturally driven to sleep with multiple partners, why would we have to make laws and religious codes to reward monogamy? Everyone would naturally behave monogamously if we were naturally monogamous, just like we have laws to make us wear clothing in public and not assault people who piss us off. It is natural for humans to sleep around, walk around naked, and get into fist fights, but our laws and moral codes advise us to transcend our basic programming. There's nothing wrong with laws that require us to behave unnaturally. I'm rather happy that we have laws against assault, and nudity on public transportation, and public urination.
We can overcome our natural urges and make unnatural behaviors standard practice, and that's OK. Unnatural is not a bad word. What offends me is when people demonize my natural urges to fantasize about other people when I'm in a relationship. That is absolutely natural and nothing to be ashamed of. There's nothing wrong with people who feel absolutely no non-monogamous urges either, but they should recognize that other people have natural urges to be non-monogamous, even if it's just a married woman's harmless fantasy to sleep with Captain Jack Sparrow. You can ask them to not give in to those urges, but you can't suppress them entirely. They are biologically programmed into our bodies, and we can't keep saying that only bad people feel the urge to sleep around.