I think that the reason men are more commonly the instigators of rape, whether towards men or women, has more to do with the fact that it is easier for them. I'm not even talking about muscle. I'm just saying that men have this gland that protrudes from the front and can be stimulated just by touching something. Women, on the other hand, need to be focused on and have a lot of facets that play into their sexual pleasure. Psychological factors play a huge role in these actions, and every perceived difficulty creates less desire for the majority of people.

http://www.nij.gov/topics/crime/rape-sexual-violence/pages/victims-perpetrators.aspx

Feel free to elaborate and/or debate