I really like this manga but...

They are more so mad at the divorce than the dad’s relationship. They just held some hatred towards homosexuals because of their father.

I kinda get you on this..

I think it would be weirder if they were to suddenly stop hating their father once they worked out that they had fallen for another guy. You like what you like, what you hate can’t really stop that, at most only hider it.
And like the earlier comment said, they are upset with their dad because of the divorce, as an add on effect that made them feel like they hated all homosexuals.
Even if their father divorced for another women they would probably still not like that other woman.

(Not agreeing with their old views but they should get their chance to change them now that they themselves have found their own lives)

I think before the homosexual nature of their dad's relationship, they hate the fact that it was because of his relationship that their family got separated. Because they were kids it would be very easy to associate this hate for the relationship in general with hate for homosexuality in itself. Now that they're growing up, I think they're coming to terms with the dad's relationship itself and recognizing that it's not homosexuality in itself that they hated. But that's just my take on things and you're free to disagree.

It's not that they hate homosexuals. Rather, they hate their dad being a homosexual bcs he broke apart the family, so to speak. But they have come to terms with the fact that their dad didn't do what he did bcs he wanted to rather bcs he couldn't lie to his family and himself. They are still a little sour about it but they don't have any hatred or prejudice towards homosexuals anymore. They misjudged for homosexuals for particular reasons. That's all.