Grab things are not as black and white as you believe. In 1980 Gibson indecently assaulted a 15 year old school girl. At the time they were not together 19 years and did not have 3 children. She WAS a 15 yr old school girl. He had no inkling of the future and certainly no right to do what he did. It was a grave error of judgement on his part.
He has aknowledged himself that it was wrong. Her father has probably more insight into the kind of man he is than you do. He has recently served two years in prison for fraud and deception, he seems to have a problem with judgement.
He isn't fit to be a position of trust, especially where children are concerned. He could just have easily ruined this schoolgirls life forever. The point being, he had no self control and no concern for her welfare at the time.