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.
Funnily enough, in the 50’s and 60’s - parents were often more concerned with their kiddos “going steady” and preferred them to date around.
Polyamory is definitely dicey without a lot of emotional maturity (tbh I don’t think most adults could handle it), but also teenage dating drama is silly. As long as she’s safe and happy, you should probably just be okay with it.