I wasn't going to comment on this because I've been in a monogamous relationship for 15 years at this point, but one thing I noticed in the comments is really interesting to me.
Pretty much everybody is saying they know they're attracted to someone physically/sexually right away, and that was not my experience when I was dating regularly. Maybe I'm odd, but I always found that most of the time I would get to know a person first and then I would start to notice things about them that I found attractive and kind of mold my way of thinking and looking at them around those things that I was attracted to in them. Sure, there were exceptions, there are people out there where I was just like - damn everything about that person is hot (my wife being one of those) - but I dated women with a wide variety of physical features and really the only commonality was that I liked being around them. The physical stuff was secondary (although still important) for me.