I honestly think that straight girls believe this because of their parents. If everyone was given their own brain to think with and their own opinions to work with our world would be a lot better.
I've realized in these past couple of years that being a lesbian in a world where people demand you to be a certain way is kind of difficult. I went to a rally for the proposition 8 campaign and an older man insisted that I was going to touch his three year old daughters vagina. -_-
I don't know who in their right mind would even think of accusing someone of this but apparently on top of being a person who is 'diseased' with being gay i'm also a pedophile?
Anyways like I was saying many straight girls are brainwashed by their parents to only assume that girls and boys belong together and not girls-girls or boys-boys because that's how they themselves were raised to know.
Straight girls are also convinced that since they have only been with a guy than that is what is right. By the way they can't base their opinion off of something they are clueless about, from my own point of view I think sex is WAY more hot with a girl because guys have penis's which to me is Blah and for two when a girl is having sex with you they get all fierce looking and titties are a plus :]] haha.
But some girls are just very ignorant that was the moral of my answer. Sorry for blabbing about shit for a while.