It seems that when the Lord was making the world, he called man over and bestowed upon him twenty years of normal sex life.

Man was horrified. "Only twenty years of normal sex life?" but the Lord was very adamant, that was all man could have.

Then the Lord called the monkey and gave him twenty years. "But I don't need twenty years", he protested, "ten is plenty for me."

Man spoke up eagerly, "Can I have the other ten?" The monkey graciously agreed.

Then the Lord called the lion and gave him twenty years, and the lion, like the monkey wanted only ten. Again the man spoke up, "Can I have the other ten?" The lion said that of course he could.

Then came the donkey and he was given twenty years-but like the others, ten was sufficient-and again man pleaded, "Can I have the other ten?" The donkey said that yes he could.

This explains why man has twenty years of normal sex life, plus ten years of monkeying around, ten years of lion about it, and ten years of making an ass of himself.