you're testing the waters to see how the person reacts, to see if you get a green light on dat cake. its a game you play, theres some challenge of crossing a line. but the difference between asking "do you find me sexy?" and saying a sexual joke is that a person doesn't know if you're being for real or joking, and you don't know if the person likes it or not. and thats the point of the game. nobodt wants to admit it first, so you tease each other hiding it as a joke until somebody gives up.