How can forgiveness even work when major boundaries are crossed within a family?

My ex used to guilt me for not liking his dad, even after telling me his dad cheated on his mom and then made a move on my ex (his son’s ex technically stepson, but still).

A year later, a lot about how he was raised makes more sense to me. I’m genuinely trying to understand how someone forgives a parent who not only cheated but crossed such a serious boundary, and then continues to act like everything is a ‘perfect family.’

His family always felt fake to me, and the entire situation still feels deeply wrong. During all of this, he always took his parents’ side. His mom chose to stay with his dad, and both of them ended up blocking their own son everywhere even after what the dad did to both of them. It honestly makes no sense to me.

Was I not being understanding enough? How does forgiveness actually work in families when boundaries are crossed this badly?