When the good Lord was making the world, he called Man aside and bestowed upon him 20 years of normal sex life. Man was horrified, but the Creator refused to budge. Then the Lord called the Monkey and gave him 20 years.
"But I don't need 20 years," said the Monkey. "Ten years is plenty."
"May I have the other 10 years?" asked Man. The Monkey agreed. The Lord called the Lion next and also gave him 20 good years. The Lion also only wanted 10, so again Man asked for the remaining ten. Then came the Donkey, who was also granted 20 years. Like the others, 10 was more than enough. Man again asked for the spare ten years and got them.
This explains why Man has 20 years of normal sex life, 10 years of monkeying around, 10 years of lion 'bout it, and 10 years of making an ass out of himself.