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.
Nothing wrong with plural groups with consenting individuals as long as there is no abuse (think Mormon polygamy).
I was in an open marriage which is not the same, but nothing weird happened.