Need explanation on that scene

1. Ok, I didn't watch this movie in ages. But I remember a scene, near the beginning, at a punk rock show. A guy says something to a girl, the girl pushes him, then he starts to undress her. Then all the people around start to undress her as well. And then, the singer of the band (don't remember what band that was) says to the crowd to stop it, otherwise they stop playing.

As I previously said, I didn't watch this film since many years, so maybe I don't remember that scene exactly as it was. But I remember finding it really weird at the time, and a little bit lame I must say. Besides, doesn't it depict a very bad image of the punks?

2. Also, when the main character goes to his first punk rock show, he drinks something, a beer I think, and I think I remember someone behind him putting something in his drink, which made him really *beep* Ok, I don't live in United States and I was still very young in the 80s, but do things like that really happen in real life? I mean, could someone you don't know and who probably doesn't give a *beep* about you, put some drugs in your drink for absolutely no purpose at all? Sounds weird to me.