In this story there's a girl who is mentally fragile in some way -- I don't remember exactly how, and I don't think it's spelled out explicitly, but there's something about her that isn't entirely neurotypical. There's also a much older man who is close friends with her and, I think, a bit of a mentor -- he's in a parental role and doesn't think of her romantically. I believe most of the story is from his POV (it's third person). Near the end of the story, the girl gets pregnant somehow (not from the older man -- I think from someone more her own age, and it was probably not entirely consensual).
She goes to the older man, very worried, and he thinks, when her family finds out, "They would take it away from her, and they must not do that, not to [girl's name]." (That's not a direct quote, but it's close to that. I tried doing google searches on variations of that and found nothing.)
Then it cuts directly to the last scene, which takes place a number of years later, and might (?) be from the POV of the man who got her pregnant. I'm not sure about that, but definitely the POV is a third character, not the girl or the older man. That person sees kids (I think multiple kids) running around -- one of them talks about "Dad" or "Daddy" -- and it becomes clear, without anything being said directly, that the older man has married the girl and adopted her kid (the oldest kid the POV character sees being the one in question). The story ends on that realization.
I read this story in the late 80's (possibly early 90's), though it feels like the story may be a little older than that. The vast majority of the short stories I read at that point were SF, so that's why I think this might be too. Possibly all of this took place in a post-apocalyptic landscape (I feel like there were forests and such, but not any technology to speak of), or might be by an author known for SF like Zenna Henderson or Clifford Simak, although I don't think stylistically it was either of those two authors.
(Also posted asking about this on the Straight Dope, but got no answers there.)