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.
In general, your emotional reaction to things doesn’t make you a bad person. How you respond to it might.
I think trying to interfere as long as there’s nothing physically dangerous or nonconsensual going on would make you a bad person. The fact that you’re asking this question suggests you don’t plan to do that.