The sound is pretty universal, not a lot of ways to mistake that, and Yuuri's not super big on porn, but he's seen enough to know he definitely doesn't want to try to deal with the spit-or-swallow dilemma right now. But he's also not sure what else to do about it, definitely isn't suave enough to like, stand up and keep pumping his hand or anything, is mostly short-circuited out, himself. So he just pulls back, shallow, more tongue and his hand moving than his head anymore, which to him seems like a reasonable solution. They're in a bathroom, they can clean up.

What Minami is doing is probably coming on his idol's face, but Yuuri still hasn't really made that particular leap of logic, and neither of them probably really have any idea that that's skirting the line of kinky. It's just kind of the thing that happens.