Recently, our daughter (17 in June) told my wife and me that she has started dating not 1, but 2 guys. No introductions needed since we already know them; they’ve been close friends of hers for quite some time now. Daughter put it as: why do I have to choose if I like them both and they don’t mind sharing?
I just don’t like it.
Such emotions are often 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.