"Optical Illusion" joke

A man and his wife have been stranded on a deserted island for many years. One morning following a bad storm, a new guy washes up on shore. The new guy and the wife are very attracted to each other but realize that certain protocols will have to be observed. The husband, oblivious to the pheromones floating around, is just glad to have someone new to talk to.
"This is wonderful! Now we'll be able to have three people doing 8-hour shifts in the watchtower instead of two people doing 12-hour shifts."
The new man is only too happy to help and in fact volunteers to do the first shift.
He climbs up the tall tower and stands watch, scanning 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.
The new man yells down, "Hey, no screwing!"
They look at each other and yell back, "We're not screwing!"
A few minutes later, they start to put driftwood into the stone circle.
Again, the new man yells down, "Heeey, no screwing!"
Again they yell back, "We're not screwing!"
Later, they are putting palm leaves on the roof of the shack to patch leaks.
Once again, the new man yells down from high above, "Hey, I said no screwing!"
They yell back, "We said we're not screwing!!"
Finally, the shift is over and the new man climbs down from the tower; the husband starts to climb up. By the time the husband gets halfway up, his wife and the new man are already screwing their brains out.
Once at the top, the husband turns around and looks down and says to himself, "Son-of-a-gun. From up here, it does look like they're screwing."