It's when one sticks their penis through the American-Mexican Border and another person inserts the penis in a body opening.
(Guy sticks penis through border)
(Woman puts penis in her vagina)
Border Patrol Guy 1: What are they doing?
Border Patrol Guy 2: It's called border sex.
(Woman puts penis in her vagina)
Border Patrol Guy 1: What are they doing?
Border Patrol Guy 2: It's called border sex.
by AFuckingMotherFucker November 22, 2011