Just been reading through all the replies on this topic and not once has it been mentioned that the person may be thinking about the comfort of the people roleplaying. I, myself would never create a rule to say you had to have x or y sexuality but you need to think what it someone isn't actually comfortable with roleplaying a same sex pairing. Or it could be there you can't assume that everyone will actually know how to write a same sex pairing mostly just because people use personal experiences or stuff they've read when it comes to writing and they might have never experienced that. That's just my thoughts though.