Because while people on this forum are totally cool, you do start to feel like a freak if you actually prefer monogamy. I had a whole coming-to-Jesus (not literally, I use that term to mean, enlightenment of a certain kind) about my views on relationships and sex. In fact, since then, my (monogamous) relationship and sex life has been quite amazing.
Anyway, when I am in love, I honestly don't notice anyone else. the hottest guy could be next to me and I wouldn't see him that way at all. That's all the reason I need.