Why do condoms come in boxes of 3, 6 and 12

A man walks into a drug store with his 8-year old son.

They happen to walk by the condom display, and the boy
asks,
"What are these, Dad?

To which the man matter-of-factly replies,
"Those are condoms, son. Men use them to have safe
sex."

"Oh I see," replied the boy pensively,
"Yes, I've heard of that in health class at school."
He looks over the display and picks up a package of 3
and asks,
"Why are there 3 in this package?"

The dad replies,
"Those are for high school boys, ONE for Friday, ONE
for Saturday, and ONE for Sunday."

"Cool" says the boy.
He notices a 6 pack and asks: "Then who are these
for?"

"Those are for college men," the dad answers,
"TWO for Friday, TWO for Saturday, and TWO for
Sunday."

"WOW!" exclaimed the boy,
"Then who uses THESE?" he asks, picking up a 12 pack!

With a sigh and a tear in his eye, the dad replied:
"Those are for the married men.
ONE for January, ONE for February, ONE for
March......... "