When fucking in a public place your girlfriend starts howling as she gets off, attracting a crowd. By the time she gets off the second time, the crowd starts to cheer her on, throwing change to show their appreciation for the show.
When your banging your girlfriend in the library against some bookshelves and security comes, so you have to cover your knob with a pepper pig book. Security see's your dilema, winks and directs you to the female anatomy section.