First, yes I, too, had the feeling that Kazuto would (probably) fallen faster for Shino. The chemistry between them is there right from the start and it has been stated numerous times that he likes to tease her very much. Well, I don't know much about guys....but, if a guy is not attracted to a girl, would he act this way? Maybe..maybe not...
And then, there is Asuna...certainly, she is the stronger girl, mentally. Asuna has never shown that she is the squishy, fragile-minded type who would let herself be bullied by anyone. Heck, she was the VGM of her guild in SAO and one of the top players.
But then again....she never gone through the life that Shino did.
Lastly, I think, Shino and her male friend, Kyoji Shinkawa's relationship is purely platonic. At the very least, the feelings are not mutual and/or not reciprocated by Shino. She rejected him pretty clearly and I believe she only agreed to go out with him after/if she wins. They aren't a couple - just like Suguha and Recon are not.
Kazuto will never cheat on Asuna. I guess they have gone through too much together so no matter how much in common Kazuto and Shino has, they cannot start it. But that's the more reason why I love them both.
and I mean Kazuto X Shino.