I had the impression that Sisko's own sheltered upbringing gave him a fairly narrow definition of family. Biological parents raising their biological children. That's how he was raised (or so he thought at the time), that's how his son was raised, that's probably how his father was raised. That's how Rugal should be raised. Because that's the right and proper form a family should take.
Not saying I agree. It was a bad decision. Just saying that was probably what Sisko thought. Or if not Sisko, it was probably what the writers thought.