@lovehydration
Week late responding to this but I don't keep up with stuff as good as I should.
The original pun is on the words 品行 (hinkou), which means moral, and 淫行 (inkou) which means obscene. So it went
品行 -> HINKOU -> H 淫行 (ecchi inkou, ie ecchi obscenity)
In retrospect I actually don't really love my decision to localize the joke to "Mmm, oral." Since the topic is about masturbation, bringing up oral into it kind of reads like a non sequitur (and then the next chapter they actually talk about oral and Miyako knowing about it even though I made her act like she knows what it is without Reika commenting already here). It's not great.
I don't know how else I'd translate it though which is why I've kept it.
(though, fun fact: "throat" is also "inkou," so Miyakonojou
could have turned turn "hinkou" into "h throat" instead. she didn't, so it's irrelevant to the chapter and the translation, but still, imagine if she had)