It seems that 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 also.
"But I don't need 20 years", said the monkey. "10 years is plenty."
Man spoke up and said: "May I have the other 10 years?"
The monkey agreed.

The Lord called on the lion and also gave him 20 years. The lion, too, wanted only 10.
Again man spoke up. "May I have the other 10 years?"
"Of course", said the lion.

Then came the donkey who was also given 20 years. Like the others, 10 years was enough. Man again asked for the spare 10 years and he got them.

This explains why Man has 20 years of normal sex life, 10 years of monkeying around, 10 years of lion about it, and 10 years of making a jackass out of himself.