What is it with you guys? And when I say guys I'm talking about Girls

But seriously. My friends & I have been talking about this...Why is it that girls are so ...uneasy about getting on someone that they don't know very well when there is a definite "spark" or mutal attraction?

Not only that, but even if you know them fairly well, they are uneasy about coming over without bringing a friend, if it hasnt gone down with the 2 of you before?
(and then the friend turns out to be a total cock-block, you ask the girl "hey come check out my room" and the other one follows! )

I can understand when there is some question as to whether or not the guy is interested, but I'm talking about..she for sure wants the guy, and he for sure wants her. Are girls really that "Worried" about being seen as slutty or what's the deal?