Here again, the situation with polygamy is diametrically different. There is ample evidence that polygamy has many severe consequences for third parties and for society as a whole, and the social interests at stake are very obviously related to a legitimate government purpose (many purposes, in fact). There’s no way the ban on polygamy could fail a rational-basis test.

Ah, but what if rational-basis isn’t the standard the courts apply? What if they find that polygamy implicates a fundamental right? In that case, “strict scrutiny” would normally apply. That’s a higher bar: the law under challenge must further a “compelling government interest” and must be narrowly tailored to achieve its goal.

The ban on polygamous marriage quite easily passes this test, too. The government’s interest is as compelling as they come, and the policy is tailored quite appropriately to fit the goal. Remember: it’s legal for a man to live with multiple women, have sex with multiple women, and even raise children with multiple women (or men!). All the government is doing is denying plural relationships the specific government benefit of a marriage license. This is a well-tailored way to prefer and institutionalize monogamy, without making private consensual conduct illegal.

It also avoids forcing the government to redefine marital relationships in all kinds of ways, because there is no existing template for polygamy.