I said in a previous post that Rachel was never promiscuous, and several people have offered evidence that said she was. Here is what I based my conclusion on:
There was one episode with Tom Selleck, where Monica had to tell Richard how many men she had slept with. Monica's number was never revealed. Rachel told Ross the same thing, and there were only, I think, him, Paolo, Barry, and one other guy whos name escapes me right now. (I'm sure the more dedicated trivia buffs will correct me if I'm wrong).
In the first couple of seasons, Monica was this capable, sophisticated type (on the surface at least), who wasn't too sentimental to sleep with someone on the first date. (This was confirmed by an early episode where the women were trying to become "goddesses.") Rachel was a former high-school tease who was a romantic at heart, and although she had gone kind of far in high school, had only actually had sex with her more serious boyfriends. This is my impression, anyway.
Of course, the characters have changed quite a bit. Monica is now barely functioning, and Rachel is getting very cold and hard.