Creator gives man 20 years normal sex life
When the Creator was making the world, he called Man aside and bestowed upon him 20 years of normal sex life. "Only 20 years?" the horrified man exclaimed. But the Creator would not budge.
Monkey likewise received 20 years. "But that's too much," he said, "I only need 10". So Man asked for and was granted the excess. In like fashion, he also secured the unwanted 10 years from both the lion and the donkey.
This explains why Man enjoys 20 years of normal sex life; spends 10 years monkeying around; 10 years of "lyin" and 10 years making an ass of himself.