A plane crashed into the sea, and the only survivors were one man and six women who washed up on an island. They were civilized people, so they worked out a division of labor for building shelter on the island, including a sex rota. Once a week, the man would have sex with each of the women, plus a day off for himself.
For the first couple weeks, that turned into a month and then two and three, all was well. As time wore on, though, the man's six-times-weekly duty began to pall on him, and he began to treasure more and more his day of rest.
One day, while he was wandering down by the beach, he saw something out on the waves, washing towards the island. As it came closer, it was revealed to be a raft! With a person on it! A male person, even! The man was overjoyed, finally somebody he could split his six-fold duty with.
As the raft washed closer, close enough for the rider to be heard, he stood up, waved one willowy wrist, and shouted: "Hey there, sexy!"
"Ah, crud. There go my Sundays!"