When someone invites you to a birthday/frat/whatever-other- kind-of-party-you-can-think-of party and it turns out to be a sex party. If you refuse to participate, you will arrested. Hence the "mandatory".
Markus: Yo dude, how was that party you went to yesterday?
Ken: Dude, it turned out to be a mandatory sex party!
Markus: Oooohh! Dude you got BURNED! HAH! So did anyone get arrested?