The very worst thing about the Uchiha forehead flick is that it is totally a symbol of postponed love. Sasuke sought for Itachi's love and attention, but Itachi only had a little bit of time for him before he was off again.
Having Sasuke repeat the action is like saying that Sakura will now be the one fighting for Sasuke's attention. What a terrible marriage for her to have! Whether she loves him or not. To be the one who's left behind, waiting for 'some other time' that will likely never come.
Naruto's recognition of her forehead was such a positive thing. A romantic moment from Naruto and a truly gratifying moment for Sakura, where she is finally recognized and appreciated just the way she is. And that he's always love her that way.
But Sasuke's forehead flick doesn't recognize Sakura. It doesn't have anything to do with Sakura at all. Instead it's a reminder of a childhood heartbreak of Sasuke's. I wouldn't even consider it romantic. More bitter-sweet.