Never stop and they never die,
They just keep on humping
How they multiply

The story:

For a while I managed a charity shop which two Mormon
missionaries attended as volunteers. As they were far from
home I tried to make them feel welcome. One day, Crazy
Horses was played on the radio and, just having a laugh, I
started rocking out on air guitar and singing along. When it
came to THAT line, a stony silence descended. We never saw
them again.