… a young, nursing mother as a capturable character? Romantically, in the sense of meeting a nice young woman with a baby and dating her, rather than a fetish game where you impregnate someone and continue to violate?
Or would this drastically violate social taboos?
I did a bit of searching for some key terms, and went through a bunch of getchu and erogamescape pages, and… I’m sorry, but it looks like your fetish is too outlandish even for the Japanese eroge industry. =P
Or I suck at searching, either way, but the only titles that came close to what you were describing were terrible, terrible fetish titles.
Yes, the first that comes to mind would be Pia Carrot e youkoso!! 2, in the person of Yamana Harue.
Not my fetish, just curious. Although discovering fetishes that are completely unserved always makes me think of dojin possibilities 
Yeah, I was kidding when I used the term ‘fetish’. Although it looks like olf’s found one-- I never played any of the Pia Carrot games myself =p
You should play Pia1, Pia2 and Pia2.5: great classical games with pre-otaku/moe fad characters that, alas, fell out of popularity. For instance, they don’t have little sisters such as Milk/Rumi any longer – which is IMO a big mistake, considering how laincursed popular she always has been. They tried to put her again in Pia Go, in an effort to appeal to the old fans but this girl is but a pale copy of the original Rumi.
Also, Misato Mitsumi FTW!
Me and an IRL friend were talking about b-games once, and I randomly threw out an example of what’s too niche for anyone to make … then it occurred to me it would actually not be a bad idea.
You have two sentai teams. Catgirls, and Bunnygirls. (Not the kind with bunny girl suits, actual bunny girls.) They’re fighting each other. It doesn’t even matter why (as if that were the point ) as long as it’s not too serious. You pick one of the characters as your squad leader (who becomes the main character) and the opposing squad leader (who becomes your arch nemesis / fated significant other). Wacky yuri catgirl-x-bunnygirl sentai hijinx ensue, with endings for each pairing (of course) depending whether you win (good ending) or lose (bad ending).
I am sure there are some around, but they never interested me enough to remember their titles. I am more into the rape, gangbang, harem, fuckfest titles.