I completly agree with you!
I don't think he isn't necessarily happy being with Yui, he just went through hell when he went back to his parents house, so it would leave anyone drained emotionally! Hence why he looked so dead inside.
But I do agree with most of what you said! I just feel as if it isn't a fake relationship to this older male. It isn't what you'd call normal love.. but maybe he can TRY his best to redeem himself with his partner and try his best to form it into something that was once terrible, immoral, and disgusting into something more healthy? You know? Like cleaning up his act so that he can finally start treating his partner right and get over his tendencies towards revenge of being a victim. Of course he will forever live with his past mistakes, but that should't stop him from changing and bettering himelf!
It wasn't a happy ending you guys. Did you see his eyes? Hollow and utterly empty. He only went back to Yui because he didn't want Yui to go through what Ojisan and he had dealt with (fucking little boys and getting caught). I think he went back to Yui so Yui could live happily, because at least that way someone is happy and won't be making the same terrible choices. You can't tell me this is a happy ending. You can't call this cute or lovely. This is a fucked up relationship and I much as I love reading fucked up shit it pains me to see a beautifully executed work get received the wrong way. Lets respect the story by praising sensei for her work, but pease don't call this a cute happy ending. If I wrote this I would be insulted by such comments