When you are on the receiving end of hearing or watching sex happen in real life.
Man, those two people are fucking in the next room and I can hear them. I'm a victim of second-hand sex.
When you have had so much exposure to another persons sex life/sexual activities, that you know things about them and/or their partner(s) that only someone who has had sex with that individual or their partner(s) should know.
"heard you had second-hand sex with your roomates girlfriend"
"yeah man, bunk beds suck."