Nick used to practice self-flagellation right after discovering his own immortality. Joe could tell it had more to do with self-hate and punishing himself than actual atonement or even coping with their newfound ability, so he offered to do the flogging himself. That way, he always gave Nicky just what he needed, never letting him take things too far, and providing him with words of comfort throughout. It worked really well for a few centuries and, eventually, Nick stopped needing it altogether.

However, something triggers all that anger and self-hate in modern or more recent times and Nick falls into bad habits, resorting to hurting himself once again. When Joe finds out, he's furious his beloved would do something like this behind his back.

They agree to go back to their old arrangement but, this time, Joe simply bends him over his knee and gives his bottom a solid spanking. It's humiliating to be punished like a bratty child and Joe is only using his hand but he's not holding back and that hurts.

And Nicky knows he shouldn't be aroused by it, but holy shit, he's getting harder by the second.

TLDR - immortal husbands accidentally stumble on new kink during punishment.

Consensual from beginning to end, please. You get a cookie if you include dirty talking.