I have a hard time domming a male. I would rather kneel at his feet, but alot of guys are intimidated by me =/ I'm really not that scary ... >=3
On the opposite spectrum, I have a very hard time being dommed by a chick.. I just can't take her seriously. I would rather see the girl kissing my feet or tied to my bed.
I think that, even with us switches, we draw a line as to who gets which side to keep that balance. Of course, there would be exceptions to every rule, but for the most part, I think it is a way for us to stay organized.
Alot of people think that switches will be a sub and suddenly turn dom in the same session. I can't speak for everyone, but I know I am not like that. I like a clear cut relationship, because otherwise, you can't enjoy that role fully.