I've seen rants on het crack!pairings though... that FFVII one made Fandom Wank. But yes, I think for many people, even totally non-homophobic people, characters are percieved as "seeming straight" much more often than they are percieved as "seeming gay."

And occasionally you get the "Maes Hughes is married and loves his family so you can't write slash about him!" business... I know many people who agree but it's not like married men never cheat on their wives with other men.

To be honest, not that many characters in my fandoms (Harry Potter and shounen anime mostly) seem "gay" to me but I don't let that stop me from slashing them. But I have a friend who is a slasher and thinks the idea that Severus Snape could be straight is completely laughable, and I just cannot relate to why she thinks that. (She also thinks Ron is straight and not even possibly bi, yet I'm the one who writes Ron/Harry. Go figure.)

Sorry, rambled a bit, but this is a topic that interests me.