@samuelmichaelsWouldn't that be the same as the 'not realizing I had a child in the first place' stories? Though, considering it, with adopted children stories in general (like in Grey Wolf's continuing epic-length Variations on a Theme) where the characters are never quite sure of their actual backgrounds. Yet if those are truly accidental pairings anyway, there's not the same level of shame as there is in a 'I just never thought/cared enough to look at their face' story.
Again, it's more of a longer story theme than a short story with a shocking ending. It's one thing to know in that moment, than it is to eventually find it out later, as how would the 'by the way, who were your parents?' line after a temporary one-night stand?
It's still a great premise, yet it's better as a slightly longer short story, where they accidentally meet up at a family reunion, only then learning they're related. I've actually written a couple of those, yet it was harder pulling it off, effectively. In one, an older man, who's dating two college age girls, is invited home to meet their families, only then, realizing belatedly, that he's actually screwed every member of the family for three generations.
Again, a great premise, yet harder to carry off, convincingly, as after the initial shock, it's hard maintaining the overall story tension though each remaining shocking revelation (the law of diminishing returns). That law doesn't apply equally to all things, as some things inherently have a lesser efficiently by default.