1I dunno, because sometimes I think fandom makes more of a point of making sure characters "switch" off much more than may be the case in real life (well, at least in slash if not in yaoi. I don't read yaoi because I don't like the ultra-ultra-strict top/bottom masculine/feminized rules that seem so prevalent there.)
When I talk to my gay male friends in RL, many of them are fairly strictly a top or bottom, or at least have very strong preference one way or another. So I tend to find it more annoying in slash fic when an author makes a point of constantly having the characters switch back and forth (for "fairness" or whatever) vs. one "primarily" being the top and the other the bottom, with some variation on occasion.
Because I do think people do have preferences in that regard, and aren't going to, like, make a chart or something to make sure each partner gets equal time in topping or bottoming.
166 tl;dr response
When I talk to my gay male friends in RL, many of them are fairly strictly a top or bottom, or at least have very strong preference one way or another. So I tend to find it more annoying in slash fic when an author makes a point of constantly having the characters switch back and forth (for "fairness" or whatever) vs. one "primarily" being the top and the other the bottom, with some variation on occasion.
Because I do think people do have preferences in that regard, and aren't going to, like, make a chart or something to make sure each partner gets equal time in topping or bottoming.