What would you do if your parents were to denigrate your idol in front of you or an outsider?

If my parents were to denigrate their favorite idol in front of me or in front of outsiders, I would choose to tell them in a more serious way. Everyone will have their own favorite things, just like they like to go to the square to dance in the square is the same, then they will also have a very favorite stars, this is everyone should be respected one aspect. Maybe it would be better to use this more communicative way.