Not something I would normally do. What's the rush?

I've had first-date sex once, and that was with a woman I (gasp) met online. I think we were both so shocked that neither of us had self-misrepresented, sex was inevitable.

Otherwise... the possibility of sex on the first date makes me vaguely uneasy. I'd wonder how often she did that sort of thing. Not that there's anything wrong with that, but it ought to be a little meaningful...