I honestly think it's this sort of "manliness" status quo and the fear of being unmanly that causes most straight men act 'manly'. It's also the reason why the people who are more secure about their manhood are more likely to act less manly. Nothing to prove, you know?
That's for pretty much the same reason that some homosexuals tend to act in the way Twilight_Knight discribed earlier. It's about prooving yourself and others that you are who you are supposed to be by behaving according to role expectation. Being "a man" is sort of a social role and it's associated with certain attributes. Out of insecurity and to proove themselves that they are men, some guys try to fulfill (knowingly or subconcious, it doesn't really matter) every role expactation that comes with "being a man". To a large degree that's caused by education and socialization and only a little of it is natural (as men for example are generally faster to jump in for competitive behavior due to evolutional heritage). To many guys, being a man means to constantly proove they are manly, it's some sort of competition. (As for me, I think manlyness is less about behavior but more about what you have between your legs, but not everybody seems to see it that way.)
With that itself I have not such a big problem, the problem is rather what's considered "manly" and more importantly "not manly" in our societies. Healthy behavior such as interest in superiority in sports seems to be a good thing that comes from the wish to fulfill role expectations but for example the fear that expressing emotions isn't manly (and thusly behavior that's to be avoided) is a sick development of our emotionally crippled societies.
Sorry do derail this again, but I figured since PowerGauntlets brought it up I felt like explaining it, because why not.