Such emotions are often can be triggered by ignorance - that’s not a negative, we are all ignorant in one way or another. What counts is what we do with that ignorance. Your daughter is in a polyamorous relationship or something similar to that. I would recommend you educate yourself about what that is, what kinds of healthy poly relationships can exist, what kinds of unhealthy ones can, and learn how to accept that your daughter is different - just like you are.

Seminal reading on the subject, which I would also recommend your daughter pick up, is The Ethical Slut.

Edit: You are not a bad person for feeling what you feel. But you are an intelligent being, capable of thought and reasoning, that doesn’t have to simply give in to emotions. How would you feel if somebody found out about something that makes you you, felt icky, and did nothing to question that emotion but simply gave in and treated you accordingly regardless of your other attributes?