Oh well there's plenty like Water Enemas, or air inflation. Though I'm quite sure these fit into a rather niche group of people so I understand the low priority of adding it if even considering.
We've had some niche fetishes added to the game thanks to popular vote (knotting, dragon furries, lactation) so might be worth adding your suggestion to our listed ideas channel! https://discord.gg/t83bwEtw