I've never liked the idea of polygamy, and not because of any religious views or societal views. Perhaps it's because I have a rather naive yet high outlook on love; I'll share my love and passion with one person and that one person only. Inb4 somebody laughs at me. I do have a lover. I have a lover of 2 and a half years and it's still going strong. Once, I did ask for permission from her to hook up with a guy and she agreed. The guy and I didn't make it very far because... it just felt so guilty. I didn't enjoy it at all. Well, I did enjoy it. I just had to stop before any guilty feelings, despite having permission, overwhelmed me. Wow, that sounds dirty.
To sum it up: Monogamy, to me, just feels... right. No other way to say it.
Edited: Just felt like mentioning that if you can make polygamy work, more power to you, I guess. Sleep around, for all I care. That's your right as a human being. I'll stick to my one, truly passionate love.