it seems when the Creator was making the world he called man aside and bestowed upon him twenty years of normal sex life.
Man horrified: "Only 20 years?" But the Creator didn't budge. That was all He would grant him.

Then he called the monkey and gave him 20 years.
"But I don't need 20 years" said the monkey, "Ten years is plenty."
Man spoke up and said "Can't I have the other ten years?" The monkey agreed.

Then the Creator called the lion and gave him 20 years. The lion said he only needed 10 years. Again the man asked " Can't I have the other 10 years?" "Of course" roared the lion.

Then came the donkey, he was given 10 years, but ,like the others said he only needed 10 years. Man asked again for the spare 10 years and again received them.

This explains why man has 20 years normal sex life, 10 years monkeying around, 10 years lion about it and 10 years making an ass of himself