Because his ego is at stake, which, by the way, is yet another reason you were absolutely right to reject him. Keep it up!
Basically, he's being a jerk to try and make you feel bad enough to seek his approval, i.e. sleep with him. It's a scumbag trick: the guys who call themselves "pickup artists" call it "The Neg", and are more subtle about it, but it's the same basic deal. Sadly, I cannot reach through the Internet and punch him for you, but there are a few ways to deal with him.
You've got two strategies here. The first is to just ignore him until he gives up. Even the most plodding, clueless doofus realizes it ain't going to happen eventually. The second is to take him aside, with the host of the party, and tell him that he needs to stop or he'll be thrown out (make sure the host will back you up on this, first). You might have to repeat this a few times.
Or, if he tries to get you alone, scream "RAPE!" and hit him with some pepper spray. I don't actually recommend doing this, but it's fun to consider.
Either way, sooner or later he'll get the point.
That happened to me once. This guy tried the "hook up move" on me, this time with my friends around. I kinda danced around him, pointing at him & sang loudly, “HE HO IN THE HOUSE! WOOT WOOT! HE HO IN THE HOUSE! The place erupted in laughter. He never tried it again. Not sure if it will work for you, but it did for me. Good luck!
Don't let him win! Bring one of your guy friends around as a defense. You'll enjoy spending time with someone who cares and just having another friend's presence (even if it's completely platonic) will deter this jerk.
If he has your number and is texting you constantly, you have the right to block his number. Don't let him win. He might even go so far as telling you that he likes you just to get you into bed. I've encountered creeps like this. As mindybindy said, a good friend will be a life preserver in this sea of creep you're swimming in.
Guys will take the hint LIKE after 50 tries. On a different circumstance I was on a online dating site, this particular guy emailed me consistently. I ignored him and he kept at it and soon resorted to sending me hate/disgusting derogatory emails. I finally blocked him and guess what? He made a different profile and even had the balls to look at my profile again. Again I blocked him. But yes...he finally took the hint I think (stop the emailing). Just take the hint guys...no is no.
You either have to keep ignoring the guy or be really direct with him, as Dan said. If it were me, (and I've had to do this several times before), I'd be direct with him; tell him I'm not interested and that it's time for him to stop. *Sigh* Guys are such assholes sometimes.
http://www.youtube.com/watch?v=8wRXa971Xw0
This girl knows what she's doing :)
Actually, that is most definitely NOT a "neg".
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