My intervention was truly in vain.
Let's forget other countries, Japanese girls hold each others hands and kiss each other on the cheeks frequently. And that's just "friendship". I know we Yuri fans make that "omg handholding" jokes a lot, but it's the least homosexual thing two girls can do. Unless this whole joke is supposed to be sarcastic. Then I'm sorry for bringing this up, I just found it a little weird. Even if it's a manga that's supposed to exaggerate things.
Kissing each other on the cheeks as a greeting is actually quite common in a lot of countries. Not in Japan though.
I don't think I know every intricacy of Japanese culture, but two high-school girls walking hand in hand in the hallway is weird. Holding hands in a crowd not to lose each other is something else of course. It's pretty easy to tell playful handholding between friends and clearly flustered entangling.
The "romantic friendship" nonsense has mostly died out in Japan, so I doubt that it's common anymore.
As for this specific example, what Hino was worried about was that people would look down on Koguma for being friendly with a delinquent. That's why the "Are they going out?" question caught her so off-guard. She didn't expect it either.