Well, if you think the purpose of marriage is to own each other, I guess it wouldn't make a lot of sense to you. For those of us who don't need exclusivity in order to signal love or commitment, it works out pretty well.
By the way, sexual monogamy is not a universal aspect of marriage in many cultures, even today. You're just used to what has been normalized in your particular culture. And if you've already determined you don't want to know why anyone would choose this, why are you reading stories about it? Is it simply to make your disapproval known? How about minding your own business?
In a traditional Canela marriage ceremony, the bride and groom lie down on a mat, arms under each other’s heads, legs entwined. The brother of each partner’s mother then comes forward. He admonishes the bride and her new husband to stay together until the last child is grown, specifically reminding them not to be jealous of each other’s lovers. — Anthropologist, Sarah Blaffer Hrdy
