Straight Edge is a personal choice that is not meant to be a joking matter. It is an oath that someone may make to themselves to not participate in recreational drug use, smoke, consume alcohol, or have promiscious sex. I think a true straight edge does it for themselves and doesn't ridicule others for not believing in the same view of this lifestyle.
waitress at the show: You wanna drink?
Heather: No, I'm straight edge so please don't ask anymore.