According to Marie Claire blogger Rich Santos (see his post for the others!):
We are all out for sex.
Sadly, sometimes this is true. A lot of guys are after one thing, and they can be very transparent about that. From the time I was in middle school, the older guys talked about conquest and “getting laid” as a measurement for how much of a man I was. This belief is passed down again and again. Luckily, at some point, I figured I’d use other things to define my manhood, or I just didn’t care about being a “man” in the eyes of other men.
We like big boobs.
Yes, we are fascinated with them-again-because they are the unknown. We don’t have them! I know plenty of guys who like big boobs; I like checking out big boobs, but I don’t know many guys who have big boobs on their list of requirements for their soul mate. In fact, there are plenty of guys out there that prefer smaller boobs.
He’s also got a post about stereotypes men have about women.