Gender roles, traditional gender roles (none of the feminist mambo- jumbo). The second point to consider would be biology and how our masculinized brains are wired in. I remember myself as a kid reaching for a toy gun and a monster truck instead of a barbie when I was as young as 5, 21 years later they are the same toys I am reaching for, but they are full-size now. In regards to men doing "gay" things, I gotta point out that you have to be a man first, and only then can you engage in non-masculine things, but it doesn't work the other way around, e.i. a warrior can be a sensitive poet.