If I had a nickel for every women who's asked, "Will sex on the first date ruin everything?" I'd be a rich woman with a lot of slutty friends.
All I know is that I still respect the guy in the morning even if he does put out on a first date! Sometimes I even cook him breakfast. But I'm not always sure the feeling is mutual.
This may come as a shock but there's something of a double standard in our culture: A lusty woman who is unabashed and unrepentant about her sexual appetites often runs into grief. Sometimes this is the result of her lover's religious upbringing, which may flick the unconscious "shame on" button after sex -- even while his intellect wants to argue it's okay. Sometimes it's due to a more universal "good girls don't" perception. Sometimes it's due to the fact that your partner's only interest in you to begin with was sexual and having satisfied his curiosity, he's ready to move on.
See, guys do not necessarily view sex in the context of a relationship. So their answer to your question might be: "Everything? We don't have a relationship -- we had sex!"
Look at your own motivations very carefully. Are you having sex with this person you've just met because you're in the mood for a bawdy romp? Or are you harboring fantasies that physical intimacy will flower into something more tender? Sometimes, of course, it does -- but don't count on it.
Casual sex is fine as long as you enter into it with your eyes wide open and your expectations low. And, of course, practice safe sex -- i.e., always use a condom.
Patrizia DiLucchio has been a nurse, model, topless dancer, criminologist, postal clerk, health policy analyst, Hollywood executive, editor, writer and index entry in The Joy of Cybersex