Also known as rape. Though some argue that hollering "Suprise" before the act of raping someone thus makes it suprise sex. a crime which is less severe than rape but is still looked down upon by; teachers, mothers, and the pope.
Chick: "Kobe totally raped me from behind. I didn't want to have sex with him. I'm calling the cops."
Lawyer: "Nah, it was just suprise sex. You were asking for it."
Chick: "Oh, cool."
When performed on your girlfriend, it's surprise sex.
When performed on an even hotter chick, it's rape.
Surprise sex is the reason that children occur in the animal kingdom. Male rabbits rape the females. Yes, really, my sister breeds rabbits and knows all.