Takuya's seems to be a little dense. Like, the idea that a girl, a classmate, someone he is kind of crushing on, is actually interested in his mother rather than him is beyond his imagination. Its also something that his ego might not like since is more around the lines of: "This girl had the gall to love my mother- and how dare my own mother steal my crush from me?"
As Freud would imply in the oedipus complex studies, this could make him feel betrayed, and perhaps experience a feeling of loss, since his crush was supposed to like him back. Therefore, the heavy weight of reality - that everything he believed it was happening - was actually nothing more than his own expectations. In other words, he got only himself to blame.
Yuka didn't lead him on, he was the one who wrongly assumed something. If he wanted to clear things up, he would just need to man up and actually ask her if she liked him romantically or not. Then, after being obviously rejected, if he kept assuming stuff about her, it would be nothing more than his own wishful thinking.
Mihoto isn't a cheater, nor a bad mother. She is a single, divorced woman and the owner of her own business. She have done anything wrong and when she get together with Yuka, it still won't be wrong, since Yuka is an adult by being 18.
Honestly, Takuya doesn't seem to be arrogant or a whiny guy, so I hope he support both of them, as suprised as he might be by the news.
Anyway, those who keep whining about this story are just trolls at this point. If you don't enjoy this story, I'm sure there are other that would fit your particular tastes.