Bad sex?

[rebbonk]

Ma and Pa where rocking on the front porch when Pa turned and slapped Ma.

Ma said, "What was that for?"

Pa said, "For 40 years of bad sex!"

Ma said, "Oh," and continued rocking.

Then Ma reached over and slapped Pa.

Pa said, "What was that for?"

Ma said,

Spoiler:

"For knowing the difference!"