An alien walks into a bar and sits next to a drunk guy and begins poking him in the shoulder.
The drunk guy just ignores him.
After a while the guy turns to the alien and begins looking him up and down.
He notices that the alien has no genitalia.
He then asks “You guys have no genitalia, how do you guys have sex?”
The alien, still poking him in the arm, just smiles!
This joke is included in the Black Book of Jokes. You can order it from here.