I think if "it's" there then it's there...no matter when you have sex. And I agree with Haxan...it's usually not the first encounter you have with the person on the first date...unless its a blind date.
Also, women just have to be careful when selecting who they sleep with. I think if you are smart and if you listen to your intuition then it's ok to have sex with some men right away. I waited a few months to sleep with this one guy I dated...and he was a douche. I should have just had sex with him right away and then dumped him...lol. I really don't think it matters. Time is just an illusion...