So you meet this girl, and instinctually you decide whether or not you would sleep with her.  98% of the time, you would.  You can't help it.  It's all instinct.  You see a hot chick and before you blink, your mind has already been made.  But just because you would, doesn't mean you have to act on it (right away). 

When you see that girl in your class, before you make a move it's imperative you wait a few days before you pounce; especially if you don't know anything about them.  A lot of guys have this habit of asking a girl out right away.  Common mistake. But now what if she turns out to be a pain in the ass?  You've gone all medieval on her ass and now like a dead weight she hangs around... So is it worth it?  That's up to you.  The younger you are, the more willing you are to put up with the bad after taste, but as you get on in years, you realize that you need to look before you leap.  Measure twice, cut once if you will.  Because you just don't want to deal with all the explaining and the drama.

Also, another reason you wanna wait a day or two, is to find out if she has a boyfriend.  If you find her hot, chances are someone else does too.  So before you charge in, check for snakes in the grass.  Nothing sucks more than making your move just as her boyfriend comes in to give her a hug...

Most importantly, realize that this advice is useless to you, because when that hottie is sitting next to you, you're just not thinking about anything rational.  So take your chances, because the gun is fully loaded.