And no, there does not have to be a sexual act, but I just do not feel like Kasumi is in love with Sakurako!
This is an example of poor reading comprehension.
But I understand now, your opinions are facts for you, just like the kiss on the CHEEK.
Let's be charitable and say that that picture shows the moment of contact, meaning you're right, it is a kiss on the cheek.
When you kiss someone on the cheek, how do you do it? Do you nestle your nose into their eyesocket and kiss next to their nostril? Or do you go around the side of the face and kiss the outer part of the cheek? If you do the former, you're profoundly weird.
https://www.google.com/search?q=kiss+on+the+cheek&rlz=1C1CHBF_enUS758US758&source=lnms&tbm=isch&sa=X&ved=0ahUKEwii_N694IniAhUPIKwKHezHCiMQ_AUIDigB&biw=1920&bih=888#imgrc=_
Notice anything? Cheek kisses are all on the side of the face.
https://www.google.com/search?q=manga+cheek+kiss&rlz=1C1CHBF_enUS758US758&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjS0NrI4YniAhUO5awKHf5jAusQ_AUIDigB&biw=1920&bih=888
In manga, this usually means either the kissee's head is turned away or the kisser's lips are drawn on the far side of the curve of the face.
So, hey, maybe Yukiko-sensei just didn't draw it very well and made an accidentally ambiguous cheek kiss. Of course, then it's just as plausible that it's an ambiguous lip kiss, and it's far from the only time they'd have locked lips, but let's grant you that it's a cheek kiss.
It's still a romantic cheek kiss. Leaving aside that if you kiss someone like that, you're basically kissing the corner of their mouth, It comes at the tail end of a conversation about what Sakurako means to her. She can't really put it into words, but thinking about it on the way home, she mentions the way she calls out to her.
I have had some exceptionally close friendships in my life. I've been blessed with that. If you ask me why I care about my friends, I might mention their loyalty, their care, their patience, their humor....
But "The way I feel when they say my name" would not be on any of those lists. That is an explicitly (and exclusively) romantic sentiment. Then she kisses her (maybe) on the cheek and says "I'm home", like a spouse returning from work. (Because that's basically what she is.) I think that was a lip kiss, but even if it's not? That is some super gay homo romantic lesbian waff lovin'. If you think that's platonic, I feel sorry for anyone that ever falls for you, because you are more impervious to signals than a faraday cage.