creative writing - How do you normalize a taboo custom in a setting that most readers would not agree with?

This setting' world is populated by witch covens that function as both royal houses and political entities. They are matriarchial, with witches had the head of families who directly control their houses and have indirect control over smaller branch clans. The concept of marriage works very differently from our world. Instead of marrying individuals, it is the practice of many houses for males marry into an entire clan. In other countries, exogamy is the norm, where individuals are chosen to concieve a child together while remaining in their birth clan. A mother's brother is considered the true fathers of their children, and spend more time raising them than a biological dad.

In our world, the concept of the nuclear household is the most stable form of family. Extramarital affairs and multiple partners are looked down on, and having multiple father's to your children is shameful. However, this world sees things in a different light. Having multiple kids by different men are encouraged because it adds more genes to the pool and builds relationships with other houses in the form of alliances, trade deals, etc.

This is a radical departure from most cultures but is a big part of the setting. How do you make it completely normal and a positive thing when the reader most likely has a different experience?