I don't think A. Robinson is necessarily privy to any information on this. The producers and writers decided these things. He may just have gone with the implication in the scripts, as other novel authors might have.
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Sine we expect to hear that Mila is his mother, had they gotten around to saying, the writers would probably have gone a different way. A big "reveal" that's totally expected is no fun, after all.
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The picture I find I've formed in my head: Cardassia officially extolls family above all else. They also are hypocrites, and take what they want, especially soldiers. So there's a bi g, thick dividing line between the respectability and purity of wives, and anyone from any other liasons. So, I think at one point, Tain took what he wanted from Mila. No love, no relationship. He would have considered her tainted, damaged goods, after, especially producing an illegitimate child. She goes back to being the housekeeper and that only. I'm over-explaining this I think. It's pretty obvious. Or there may have been a relationship. I have the feeling that marriages are important in Cardassia, with actual love seeming like a weakness though. Without marriage, a woman was expendable.
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Was Tain married?
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I think it was a very sad story, and that Garack doesn't know it. I think that to him, Mila was a very special figure to him, a maid who was very good to him growing up, like an aunt. The real relationship would have been hidden. Maybe Garack guessed when he was older and confronted Tain about it. It seems that Garack thinks Mila ought to concern Tain more than she actually does.
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I'll guess that the writers explained this enough for us to guess the details well enough, without losing that slight question mark or mystery.