I'm a girl too. If we say we want to stay friends it means "I don't want to have sex with you but I think you're a neat person. Please pretend I have a penis too from now on and don't be a baby and not talk to me because I don't want to have sex with you."

Nothing's worse than when guys act like this because it says to us, the only reason we're worth talking to is because there's a chance we might put out. If you liked a girl as a person to begin with, continue to treat her as a person and not a vagina. Who knows, someday she might change her mind about you. I've seen it happen.