I have a personal distaste for making either partner an exclusive top or bottom. I like to see equality in my m/m slash relationships and I don't like to see male characters get overly feminized, which seems to happen more often when characters are cast as exclusive bottoms. I just find that limiting. They're both men, they both have dicks, they should both get to do the fucking, IMO. Then again, I don't have to have penetration in my m/m slash to find it hot and loving, either. *shrugs*