Chuck Palahniuk. Suffocation (№ 277581)

She says that when mating two dogs, a boy and a girl, the head of the penis of the boy swells, and the vagina of the girl shrinks. Even when this is all over they some time can not remain locked. They have to wait - pathetic, helpless - until it's all back to normal.
Most marriages, mother says, are based on the same scenario.