Do your step-kids think you're immoral for marrying their parent?

One of my step-kids just implied that I was immoral for marrying her father. (albeit, arranged). She said her mom was going to hook up with her father again when he married me instead.

She thinks I'm immoral for having married a man that's been married before.

Has this happened to anyone else? She's 21 and I think she's a bit late to start complaining.