I actually never bought into Dorian's line "What my father knows of me could barely fill a thimble," and neither does Dorian himself. If he actually believed that then the Nightmare demon would not have taunted him with his father in the fade.
Something I'm curious about is how Dorian's parents found out about him preferring the company of men, as he puts it. Did he just straight up tell them (do people of Thedas actually "come out" to their families?) or did they find out in some other way? Maybe he was caught in act, so to speak. I know sexuality isn't a big deal in Thedas, but it is a big deal for someone of Dorian's rank in Tevinter, unfortunately.
Because of how homosexuality is viewed in Thedas he may not have felt the need to hide it at the time. Even for the elite it's not a big deal as long as it's for pleasure only, nothing more. If Dorian had played his role and carried on with other (noble) men, or slaves, in private, there would have been no problem.
It would be interesting to know when, for the young Dorian, it became more about wanting an emotional attachment and less about sex. This is probably when he realized there was a problem. You can think back to how the relationship with the Inquisitor develops to see how it might have gone. There is a physical attraction there yes, but the friendship develops first, then comes the possibility that it could be more intimate, and then they finally have sex. What does Dorian say afterward? They've had their "fun" and it could be left there, even though he wouldn't be happy about it. He's done this before and knows how it goes. I think it's a real testament to just how much he likes the Inquisitor, and also how much he wants the emotional connection, that he opens up as he does about everything in that scene. This is also the reason I prefer the sex version over the non-sex version.
I imagine that the first real crush Dorian had must have been emotionally devastating as he realized that he could never be with the person he wanted. At this point Dorian would understand that these feelings were incompatible with the future his father had planned for him and might have "closeted" himself. Not necessarily hiding the fact of his homosexuality, but not letting on that, for him, it's more than just a quirk and his preferred method of sexual pleasure. Dorian is not naive. He would have known this would eventually lead to conflict with his parents, and his father in particular.
What I think leads to the "What my father knows of me could barely fill a thimble" remark, which I regard as hyperbole, is not a lack of general knowledge of his son: his personality, his morality and principles (similar to Halward's own), but rather a lack of true understanding of the core of Dorian's being: the desire to be true to oneself, Dorian says as much during the post-tavern dialog. Halward himself bit the bullet and married, and had sex with, a woman he despised, so why shouldn't Dorian do the same? This is what Halward doesn't understand when it comes to his son.
It's interesting though. You have to be raised a certain way to believe that your own happiness is important. I'm not talking about being a spoiled rich noble, but about being loved by your parents and truly believing that they would want you to be happy. If Dorian didn't feel this, or placed priority in tradition and family legacy over his own happiness, then none of this would have happened. He would have gotten married and been miserable, and we would never have met him.
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Sorry I get carried away in writing these things.