I have never cheated and will never condone it and I don't believe Sagara thought of it as cheating nor did he do it intentionally.
I think Sagara may have thought it was time to move on. Sure you can be in love with someone but when that unrequited love bears no fruit, after all those years, do you suppose everyone will want wait until till the tide changes it's current? Or is it time to try something new for once. Yes they were in love but neither of them said they were technically exclusive. I'm not enthusiastic about his decision to bring another person to his place with Aoi outside, but I understandhis reasoning a little more.
Yeah - just a miserable premise, and sad, but it's also about growth, change, and ability to move past all that. I just don't like my entertainment to involve something with realism. ;) I'll likely re-read this story again a few more times in the future because it suits a passing mood...
People just do messed up things to other people in their lives, not just this kind of man-to-man (boy) scenario. There are men to women (girls) and women to men (boys) -- and what happens? For a while, they will commit to each other, but the feelings will burn out and then they'll go their separate ways. A guy who can claim his 13-year-old-first-love came true but then sleep with other people while he's sleeping with said first love is someone whose feelings are too shallow, in my experience. I really didn't want to read a realistic story like this - I wanted fluff and romance.