Sun is out on the beach hanging out with the S/M Female Swimmer.  Usually confident and flirty, she is actually very easy to embarrass today.  On this occassion, she's on some medication (or under the influence of a mon) that gives her a very short bladder for the day, making her usual confident self a lot more vulnerable, so much so that even touching her in public would have her squirming, else risk having her pee.  Sun can't help but to take advantage of the Swimmer's state to embarrass her (groping her belly midconversation with friends, boob touching in front of Pokemon, slapping her ass while walking in public, pulling her panties down) and make her pee in the open, with each incident making her pee more and more.

It all accumulates until they are alone on the beach, with Sun revealing he did this on purpose for some alone time.  Which then they fuck, with her most likely peeing mid-coitus, and maybe even him drinking some of her pee to prove that he loves the kink to her embarrassment.