I have another Kokuzan headcanon that I can’t be bummed putting on the chapter for it:
When they go doggy and Kokushibo is on top, he likes to put his head to Muzan’s right bicep—where one of his many hearts are—and tries to pound him to the rhythm of his heartbeat. Kokushibo just likes having something to base his speed off of, and Muzan’s Heart is easy to listen to.
From what I also believe, Muzan has a heart in his abdomen and throat, so when they snuggle, Kokushibo likes to place his head in those places so Muzan’s heartbeat can help him sleep. I’m tired and I wanted to share this ;-;