When God created man, he gave him 10 years of normal sex life.
"Is that all? I need more than that," whined man. "That is all you will get," boomed God.
God then created the monkey and gave him 10 years of normal sex life. "But I only need 5," replied the monkey.
"May I have the other 5," asked man. The monkey said "Sure."
God then created the lion and also gave him 10 years of normal sex life. "I only need 5," the lion replied.
Man asked, "May I have the other 5?" "Sure," said the lion. God then created the donkey and also gave him 10 years of normal sex life. As twice before, the donkey said, "I only need 5."
Man spoke up again and asked if he may have the other 5 and the donkey gave them to him.
This explains why man has 10 years of normal sex life, 5 years of monkeying around, 5 years of lying about it, and 5 years of making an ass of himself.