A man and his wife had been stranded on a deserted island for many years. The morning following a bad storm, a new guy (Homer) washes up on the shore. Homer and the wife are very attracted to each other right away, but realize certain protocols will have to be observed. The husband, however, is very glad to see Homer there.
"Now we will be able to have three people doing 8-hour shifts in the watchtower, rather than two people doing 12-hour shifts."

Homer is only too happy to help and in fact volunteers to do the first shift. He climbs up the tower and stands watch, observing the ocean horizon for any ships. Soon the husband and wife start placing stones in a circle in order to make a fire to cook supper. Homer yells down: "Hey, no fucking!"
They couple looks at each other and yells back: "We're not fucking!"

A few minutes later they start to put driftwood into the stone circle. Again Homer yells down: "Heeey, no fucking!"
Again they yell back, "We're not fucking!"

Later they are putting palm leaves on the roof to their shack to patch leaks. Once again Homer yells down from high above: "Hey, I said no fucking!!"
"We said we're not fucking!!"

Finally the shift is over and Homer climbs down from the tower and the husband starts to climb up. He's only half-way up when the wife and Homer are screwing their brains out.

Once at the top, the husband looks out from the tower and says: "Son-of-a-gun. From up here it DOES look like they're fucking."