I think it works for the books. I'm not an expert on relationships in RL. I've had one true monogomous relationship...we've been together for 11 years.
I am open to understanding other relationships. I also have a romantic notion of Beltane bonfires, and fairy festivals, by which your 'partner' was found by firelight to celebrate the fertility of the land. Any child born of those festivals, were the villages children, loved, and thought to bring luck.
I believe that sometimes we are too rigid in our thinking, about relationships. What works for me, does not work for everyone.
I love my husband enough that I could share him with another woman, as an equal. He, however, would not share me.