I think it's all a matter of "timing"
The vast majority of the time when you meet someone, things proceed step by step...
1. You figure out each other's name, hometown, major, all the normal details.
2. You decide whether or not that person is "attractive" to you. I don't mean in a snotty "I demand supermodel girlfriends" kind of way, I mean whether or not there's chemistry.
3. You spend some "bonding" time together. This might start at a traditional "dinner and movie" first date or even just a conversation in the corner of a bar or at a party. This is the "getting to know them" part. Building emotional connections.
3(a) You get to know each other better and form a stronger emotional connection (this step continues, indefinitely)
4. You get physically involved. A kiss. Then more kissing.
4(a) You get more physically involved. Stuff your mother doesn't want to know about.
The prpblems arise when you rush to step 4 (or actually step 4(a)).
For example, if you were friends with the other person for awhile, then it often makes sense to get to the fun stuff in step 4 earlier. You've already established a foundation.
However, if either person is just hooking up without the emotional attachment(skipping 3 and 3(a)), things can get out of wack. The connection is purely physical and that rarely is a solid foundation.
Even if you were friends first, you still need to establish some emotional connection in the romantic sense to lay that foundation.
These steps aren't defined by time, they're defined by what's going on in your head and your heart. Some couples will take weeks, months or years before they feel ready to enjoy the full potential of step 4(a). Others will meet and feel an intense emotional connection that day.
The trick is to not rush ahead of the other person - if you're feeling an emotional connection and they aren't yet, then if you rush into hooking up, it's going to be really hard for them to build that emotional connection later.
The truth that is so hard to admit is that we KNOW when it's just sex, and when it's something more. James is right, sometimes it IS appropriate for things to move pretty fast. Those are the stories you hear where they had sex the first night and then dated forever and got married. But a lot of the time, anything really early (i.e. getting nekkid first date), is not the emotional connection type.
Regarding the Lewinsky type of sex in the first night, in my opinion, that's even MORE likely to be purely physical. That's where the girl refuses to have sex (knowing intuitively the lack of emotional connection) and then offers that as some kind of backup option, hoping that it will make the guy happy.