EEEWWW! It is still disgusting.
A man who marries his girlfriends adopted daughter is still a scum bag.
The fact that he was in her life as a father figure for years comes into play. This was not just some girl he met and fell for. He was acting as the father in the family. What was in his mind all those years he was sleeping with Mia and telling bed time stories to Soon-Yi? I am guessing it was not what a normal father would have in his mind.