I'm struggling to think of one earlier than "The Camping Trip" by GoblinBoy. Obviously "School Dreams 3" had at least four thoroughly exclusionary paths. Was that the beginning of this particular technique in erotic gaming?
I don't remember the names, but most of the erotic games I played in the early 90's already had this king of mechanism. So no, it's not the beginning in erotic gaming.
Also, the technique is far to be something that came with gaming. A 1930's play (I forgot its name, sorry), was depicting a trial and wrote in such way that part of the jury was chosen among the public. Therefore, every representation was kind of unique. While there isn't a infinity of possible ending, the play had probably at least three possible ones (guilty, not guilty, undetermined).
Globally speaking, we can assume that the moment literacy started to be available for public (even if it was a small part of it), some authors searched a way to write stories offering more than one ending. We know that, during late middle age, some fortunetelling techniques
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. While not being intended to tell a whole story, it still rely more or less on the same kind of mechanism than modern Choose Your Own Adventure.
Therefore, it's to be expected that some people, between Ancient Greece and the 1930's play, tried to used this kind of mechanisms for effective stories. They just weren't popular enough and never made it through history. And, while not strictly being "two or more ending stories", there's also all the cases of satirical political parodies with a two level reading, like LaFontaine fables by example.
This said, searching for references on the fortunetelling techniques, I found this manuscript with a
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. Apparently it was wrote by
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in 1891. So far, at my knowledge, it's the first example of multiple ending story, and so the beginning of this technique.