Ya Feelin' Good?
About yourself, I mean. Sex and self-esteem cannot be separated. A one-night stand says, "I don't care about you and I don't care about me." Even if you think you're alright with casual sex and your partner claims the same, sex without commitment is still about using someone. Using others is selfish and being used by others is deadly to one's self-image. There's no better feeling than having something that others want, so don't give it up too easily.
Play Misty For Me, Mister Goodbar
Many of you may not be familiar with a couple of 1970's movies; one about a woman's one-night stand with psychopath and the other about a man's. Suffice it to say that it is just plain stupid to have sex with someone unfamiliar to you. Not to suggest that you'll end up in a dumpster, but sex can get your life tied up with some real fruitcakes. You must get to know your sex partners and that takes time, plenty of time. You don't get to know someone in a couple of dates.
Trust Me
You should only have sex with someone you trust. After all, sex, really great sex, involves losing your inhibitions and making yourself completely vulnerable to another human being. That can only be done with someone you trust and trust takes time. Are you starting to see a theme here? When sex happens too early then it becomes about acting uninhibited and presenting some notion of yourself that you believe the other person wants to see. That is self-conscious sex and that ain't good sex.