I was thinking the Sasha-Chan one, which were you thinking?
Well, besides that one, there's also at least other two series following a similarly cursed pattern.
Like the one with this shady guy that adopts 3 slaves to raise them as members of his harem but actually his actions are the ones of a decent and caring guardian/parent and the girls grow to truly like him because of it... But the series actually has some porn stuff, in the form of a really cursed prologue and some more wholesome nsfw chapters later on when the girls are older and all horny.
And then, there's the Ragged elf and the Apothecary series. Where a guy helps to heal a battered elf slave... There's a "not-canon" (according author, but it really is) doujin called Elf's past about the awful shit the Elf suffered before the canon wholesome series started