Little Johnny is passing his parents' bedroom in the middle of the night, in search of a glass of water. Hearing a lot of moaning and thumping, he peeks in and catches his folks in The Act.
Before dad can even react, Little Johnny exclaims "Oh, boy! Horsey ride! Daddy, can I ride on your back?"
Daddy, relieved that Johnny's not asking more uncomfortable questions, and seeing the opportunity not to break his stride, agrees.
Johnny hops on and daddy starts going to town. Pretty soon mommy starts moaning and gasping.
Johnny cries out "Hang on tight, Daddy! This is the part where me and the milkman usually get bucked off!"