Sometimes you bite to show affection, you haven't much with Sukuna, besides when you get more intimate, but even then it's not much.

You were sleeping with Sukuna, when he moved and slightly woke you up. You, half-asleep, moved to bite his hand that was on your face. He grumbled, before leaning over a little to bite your shoulder in response, not too hard.

"Go back to sleep." Sukuna grumbles, putting one of his arms over his eyes.