I know ppl accept this as a fact but I can't help but realise that he says "I only do that with my wife" then his wife proceeds to do exactly that with someone other than her husband.
And yes, it being another woman does count.
I mean, he comes from standard normie Japanese society where monogamy is the default option and anything else is extremely uncommon and weird to the vast majority of people. She comes from yuki-onna society where they literally need to feed on human semen in order to survive, so being highly sexually liberated isn't just cultural, it's like, an absolute necessity. And even if she commits to not feed on any man other than her husband, the culture of intimacy that completely permeates their entire society isn't just gonna go poof and become standard human monogamy culture in the blink of an eye. Also, like, look at the lesbian couple. They are apparently in a committed relationship with each other, but they still gotta jerk dudes off on a regular basis as a basic survival necessity. But rather than get jealous and complain about it, they just accept that this is how their biology and society works and mentally separate romance from the physical activity. I see Ginka's physical relationship with Suzuna in a similar way, they are close friends and they do physical things to each other as a way of being intimate in that friendship, but it's clearly separated from romance (at least on Ginka's part). Romance means sex, but sex doesn't necessarily mean romance in yuki-onna society, is how I interpret it.