I don't think Kou is being insensitive. If anything, she seems conflicted. Having someone as beautiful as Nene confess to you is bound to make anyone feel flattered, and Kou seems too inassertive to turn her down properly. Still, by not giving her a clear "no", she's kinda stringing her along.
That being said, fuck that photography-senpai! She has been nothing but purely manipulative up til now, and while I'm not entirely convinced that she had anything to do with Nene breaking up with Amane (Nene didn't seem to know her at all, and the fact that she's fallen for Kou makes it seem like she really did stop loving Amane, even if the reason is unclear), she is very obviously manipulating events right now to her own advantage and for her own amusement!
Really, at this point, it seems like Amane is the only character who is one hundred percent blameless in all of this and it's killing me how she's being treated!