penis on belly
When a character has their penis on another character's belly, or their penis is laying on top of, or otherwise making contact with their own belly.
This is often seen as part of foreplay, rubbing the penis against the belly before penetration. This can also be the primary sexual act unto itself, rubbing against it until ejaculation occurs (alternatively, the character whose belly is being "used" rubs their belly on the penis). A character who is overweight, or has a big/huge belly for any reason (e.g. stuffing, vore) may masturbate by rubbing their own penis against, or humping their own belly.