Every woman has a quietly persistent inner voice that tells her not to sleep with a man on the first night, especially if she really likes him. We try to hold out and hold off, not because we don't want you, but because we want more. We want to see you again, and another time after that, and after that.
If you're looking to have a one-night-stand, ignore this advice. As we just mentioned, we never condone lying to a woman. But, considering that you've asked this woman on a legitimate date, and you've had a good enough time for it to come to the point you might actually have sex, you probably have pretty good chemistry with this girl. Hell, you might even be starting to like her.
If this is the case, and she gets to stuttering about how she "doesn't ever do this," silence her gnawing fear that giving it up means giving up the possibility that anything might happen between the two of you and tell her the truth: "I want to have sex with you because I like you."
Nothing will silence a woman's reservations about first date sex than a man's assurance that he will not lose respect or interest in her for giving it up. But just a word of warning, if you say it, you better actually mean it. If not just for honor's sake, to avoid the headache and stress (and stalking and crying) that will undoubtedly ensue if you deceive her just to sleep with her. We've seen it happen before. It is not pretty. Plus, you know better, right?