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Some very interesting ideas here. If you'd let me, I'd like to elaborate further upon those. Keep in mind, these are the barebones ideas and specific dialogues or reasons as to why MC happens to listen to them could differ depending on previous choices and routes taken (Love vs Corruption). 

So, picture this. Since Noriko knows MC is doing Yasuka, but she's been shown to be a bit envious of this (in Yasuka's love route), we can infer it to make sense if Noriko could be the first one to suggest having a Threesome, as a way to try and spend more time with MC even if it is shared. Asami could also get winded up in Noriko's "Harem plan" (if you know the reference) and start having Threesomes with Noriko and MC as well, and maybe by accident discover a Noriko+Yasuka+MC situation. THEN, Asami starts wondering if it doesn't feel weird to have the mother involved, and asks upfront Noriko about it. To not let the plan fall apart, Noriko tries to convince Asami that it's normal for mothers to help their daughters with sex education or something, and that makes the seed of doubt sprout in Asami. 

At a parallel story arc, Sayuri and Yasuka also know each other and could even be on good terms (since this is a small town-city, makes sense that all the mothers meet often-ish, perhaps while buying groceries). This devolves into Sayuri asking Yasuka for help regarding one or both issues: a) how to deal with the loneliness since she takes care of Asami by herself nowadays; b) advice on making sure her daughter doesn't make mistakes with her new boyfriend/sexfriend (MC). This further devolves into Yasuka, after experiencing the Threesomes, suggesting/convincing Sayuri to try it as well, of course after talking with Asami first to see if she'd be cool with that. Since the relationship between Asami and Sayuri doesn't seem to be bad, and Asami already has the curiosity awoken in her (and also could believe that maybe this way her mother would stop trying to act all prim and proper and meddlesome), this can develop to the Asami+Sayuri+MC situation.

Of course, player agency is also important so his choices matter. When he hears about this development, he can either decide that this is not his thing, or follow two different paths, the Asami Top Sayuri Bottom Path (or DaughterDom Path) or the inverse (MotherDom Path). In DaughterDom, Asami takes the initiative to drag Sayuri into their relationship to see if that way she'd stop nagging her or if she can use this chance to establish a better power dynamic with her mother (maybe influenced by Noriko's advice). In MotherDom, Sayuri is the one that "accidentally" (or more directly) interferes in her daughter's eloping and manages to push the boundaries and take initiative, under the pretense of "teaching" or supervising that Asami and MC are doing it properly.