To all the people saying he's getting what he deserves... I get the ethics and moral repercussions of dating a minor.... in real life. I dunno about anyone else, but I get enough reality in reality. As I read this story, I just want the involved parties to be happy; MC, Koto, Miya... and if that means Koto and Kousuke needs to end up in a relationship, then that's what I want to happen. That's the magic of a fictional story; it opens the potential possibilities of things we could never see or experience within the normal flows of our lives, and I think that's just fine. Would I like my teenage kids(or when they get to be teens) to date a 30+ year old men? Of course not. But that doesn't mean I dont' want to see it in some story I read, so long as the writing makes me care about the characters enough.
It's not exactly the same, but its just like death; I might want to see someone die a horrible death or be killed in some story, or a game like Call of Duty for my own entertainment, but it's obviously not anything I want to see in real life.
I'm also not saying you CAN'T say 'he deserves what he gets for having a relationship with a minor". You're totally within your right to think he's a scumbag, weak-minded, etc. for being involved with a minor, just don't criticize us who want to sympathize with him and his situation.