Well, sounds a bit odd but Sakura not even able to speak up infront of Sasuke speaks tons about their relationship. She hits Naruto, scolds him, goes tsundere with him because of the comfort level she shares with Naruto. If its about opening up, I doubt whether she will even think once if its Naruto she is opening up to. But for Sasuke, she would think thousand times. She just cannot be the way she is(her real self)with Sasuke because of the dynamics they share. Its like, she is just not comfortable with Sasuke. She just cannot be herself with....Sasuke. If Sasuke acts like a pervert I seriously doubt she would even touch him but if its Naruto, we all know XD. Its the difference in their respective dynamics really. Even Sai for that matter is closer to Sakura than Sasuke. Practically, she is just not that close to Sasuke as she is with Naruto, emotionally. And that's why this difference of dynamics will occur. She can hit Sasuke or fight with him if it is an actual fight, but when it comes to hitting him 'just like that' as with Naruto, she would not do it. Rather she would not even think about it. I really cannot put my answer in words for this question but if its possible for you just observe Sakura's behaviour patterns with Sasuke and her behaviour patterns with Naruto, individually. You'll see she is more out going, opening up or to sum it, being her true self when she is with Naruto. But when she is with Sasuke, things don't really 'click'. And it is this factor which monitors Naruto and Sakura's interactions. Just try and observe her behaviour patterns with both of them individually, if you have time. I am sure you will figure out the answer yourself
Hmm ok I can understand that because that does make the most sense when it comes to each of their dynamics with each other. I guess I'm just wanting Sakura to give Sasuke one good punch in the face in this series at Least once!!! I'm being a hypocrite TvT lol