I know my personal gut reaction to this question is, "Well yeah!". But looking at it closer, it's actually very difficult to even define what extreme sports are. For this reason I'm only going to list a handful of extreme sports done on land. Air and water sports will have to wait. Regardless of where they take place, they all seem to vary in different levels of danger. Is this the only factor that makes them manly? To make it easier to discuss, let me break down the question "Are Extreme Sports Manly?" into the following four questions:
1.) Which extreme sports are manly?
2.) What makes them manly?
3.) Which ones are not manly?
4.) Why are they not manly?
Here's a very small list of the more common land ones (feel free to add your own):
Rally car racing
I am a skteboarder and don't do any other sports, so I only can speak for skateboarding as an extreme sport. I think it is manly, but not because you stand on the board and do some stupid tricks. It is manly because you try it as often until you land a new trick. Sometimes this takes 2 hours, where you fall down and have to get up. Of course the sport isn't automaticly manly. In my local skatepark is a 21 year old skater, who dresses like he is 14. It's like every sport; I think you can do it in a manly way if you want.
How about skydiving? Do you find it manly?