They never lived together, Allen had a residence nearby which he never gave up. And he always took the opportunity to publicly state that fact.
Soon Yi was probably well aware of the fact that her mother broke up the marriage of her true adoptive father, Andre Previn. Mia was friends with his wife Dory and honored that friendship by getting pregnant with Andre's child. Andre is not only a great conductor. Andre and Dory were a sucessful movie songwriting team, with Oscar nominations to prove their talents. Dory went into a tailspin when her marriage broke up. I remember her on the Mike Douglas Show singing her song "Beware of Young Girls" and she didn't hide the fact that Mia was the inspiration. When a child sees a glamorous mother betraying a friend and suffering no ill consequences, it must color her view of relationships.
Add to this Mia's actions when Roman Polanski, who directed her in "Rosemary's Baby", was arrested for drugging and raping a 13 year old. She wrote a charater reference to the DA. I guess she is only against underage sex if it involves her immediate family.
Sometimes karma bites you in the ass