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This probably is just an ambiguously romantic illustration of implied sex. ZUN's art is sometimes hard to read on top of being astonishingly beautiful, making it all the more interesting to look at!
However with Keiki's attitude and expression, I also like to see this as her being pinned against a wall by an unknown assailant and she's like:
"You sure you want to go down this road, with the Cake of the Primate Spirit Garden?" before whooping his ass.
