It's not a function of being straight, or gay. It's a function of what arouses the person viewing the porn. So saying straight porn would "do it better" for straight people is kind of senseless. Granted, the straight person would make sure that it's the opposite sex in the porn, that's what they prefer. But I don't think it would be fair to assume that porn in any form cannot be enjoyed by an audience it wasn't intended for.
Yeah you have a really good point. I guess it's all preference, but it seems like the stereotype is that straight men love lesbian porn. Almost every one of my girl friends likes yaoi and gay porn (which I don't understand why, but again, personal preference) which I guess is the opposite of a dude liking lesbian porn.