Not gonna lie, when I was reading I was like "Taiga is still an asshole" but then he had such a charcater development and then that scene between him and Kaoru during the class reunion, I was already all in. I think this is what I enjoyed the most, he went from a total asshole to a devoted partner, I'm glad I didn't give up.
Other thing that I thought by the middle of the story was: wow, Yuka was kinda of an asshole to Kaoru too, they were pairs for about 10 years and they only had sex during his heat AND Yuka still considered him just a childhood friend? Their relationship had almost zero development since middle school (so maybe it was even more than 10 years). Thank fuck Yuka realized he loved Kaoru when he thought he could lose him to Rin! Another character development right there!
people complaining about the age gap when they didn't do anything wrong or sexual are just (honestly) annoying. So far, they are friends. And it wouldn't surprise me if the author made the older guy wait for the younger guy to be an adult (20 years old) before they tried anything sexual. This seems like a very slow paced story, so that development wouldn't be weird. So, let's just not make preconceptions about things that aren't there.
( ̄へ ̄)instead of "we should break up" he should have explained the situation because from what he said, it really sounded like he was cheating, instead of the real situation: someone kissed him without consent when he was sleeping. If the other guy never said anything to Hisaka, Yukio would still be seen as someone that was cheating (even if Hisaka said he was choosing to trust him).