Moshe rose, I hate to break it to you but it’s warped people like you who are causing problems in our society. Like oomis said, it doesn’t mean he didn’t know what she looked like. I means that when he saw her, he did not see solely her physical beauty. When he looked at her he saw her as the total inner person she was and thus assumed that her beauty emanated from within.
That explains why he only realized her physical beauty when he saw her reflection. A picture, or reflection will not radiate the same chein that is seen on a live face. Thus he understood that she was objectively beautiful.
And anyway, to get beyond the simplistic, widespread definition of tsnius, tsnius is the ability to push past external trappings and focus on the essentail core of something. According to this understanding, it is quite clear why tnius between avraham and sarah caused Avraham not to realize sarah’s physical beauty. He saw her as a whole person, inside and out. Her physical beauty, to him, was simply a reflection of her inner being.
I find it hard to believe that “al kein yaazov ish es aviv vies imo vedavak lishto…” means that one should have minimal contact with this wife. It smells very unjewish.