Line? Where?!

Apparently I'm the only one that thinks I'm funny. Mostly because there seems to be this invisible like that says when you're not supposed to say things. It doesn't make a whole lot of sense to me because if I can't see the actual line, how would I know if I crossed it? Not only that but the line is all a matter of opinion! What I might think was to crude for a joke someone else might not! It just puzzles me when people get mad about it.