Responses
Hmmm i would have to disagree. You have to remembered that Woojin aka si woon is a pretty powerful guy. No one would dare bite him for fear of what he might do & every1 in school knows woojin is not to be messed with. So if he wants to put his finger in some1 mouth i doubt there would be any1 other than taemin who would be brave enough to bite him. So the fact that taemin doesnt do anything really doesnt say much.
It took me a couple of re-reads to catch this, but I believe there's significance in the fact that Si-Woon rubs Tae-min's lips and sticks his finger in his mouth.
I don't think it was necessarily meant to be a romantic gesture (at least not exclusively), but more-so to further "confirm his reaction". Just as Si-Woon begins to touch Tae-Min, he says "I wanted to confirm your reaction." Yes, he was talking about why he didn't tell Tae-Min about the CD, but I think touching him as the opportunity presented itself corresponded with it perfectly.
Listen, Tae-Min was about to get raped, beating up 5 guys single-handedly, and hasn't let anyone move on him physically without fighting back. Si-Woon is verbally teasing him at this point ("Is it because you're scared of the video getting spread around? No, that can't be it. That's not like you." *slides finger into mouth*).
So, why then, if he really WASN'T Tae-Min, would he allow Si-Woon to continue to rub his lips and even stick his finger into his mouth without at least biting him to get it out? Even if the author didn't intend that to be interpreted that way (which I honestly think they did), that's exactly what I think that was: a test to see his reaction.
Only someone with that attitude, who wasn't Tae-Min, would have bit Si-Woon right then and there, and Si-Woon knew it.