Can't remember this joke properly, here it is roughly. Bob is the sole survivor of a plane crash, he is washed up on a desert island. Over the course of about a year he gets used to island life and manages to find food, shelter and entertainment to keep him sane. One day he awakes to find a woman stood out by the sea. Is it a mirage? Bob runs as fast as he can to find Shakira stood there, she explains how her plane crashed. Inevitably over the next few months Bob and Shakira form a loving relationship, they would both do anything for each other. However one day Bob is very depressed, Shakira asks him, "Whats wrong Bob?" "Nothing. Its just.." "Anything I can do? I'd do anything for you." Bob asks Shakira to put his clothes on and draw a fake moustache so she looks like a Dave, Bob's old best friend. He then tells her to walk around to the other side of the island, and he will walk round the other way until they meet. So they both walk round and Bob shouts, "DAVE! HOW ARE YOU MATE!" He runs up to Shakira and starts shaking her hand. He then says, "You'll never guess who I'm shagging."