>>2027014my take is that it's because of typical hentai "If it's pure it must be corrupted" mindset. Combined with the fact that Keiichi is pretty much your standard "nice" male protagonist without any appreciable sex drive (even before the fact that he was essentially magically neutered was revealed) means that ntr is pretty much the only way to get her to have sex with someone.
heck, whenever it IS Keiichi having sex with her in a doujin, a majority of the time it's that he's been affected by some potion/spell/demonic possession.
