Because there's this dumb idea that people have that think because you're not the "submissive" person during your gay oral sex session, that it somehow completely absolves you of being "gay" (or at the very least, bisexual). The rationalizations for this are also quite illogical. "You can simply pretend that he's a girl". Yes, and I can also pretend the deeply religious aren't gullible morons, that doesn't make it any less true in the fact of facts.