"... is subject to prosecution at the limitless discretion of local and State prosecutors, despite a general policy not to prosecute religiously motivated polygamy. The court finds no rational basis to distinguish between the two, not least with regard to the State interest in protecting the institution of marriage."
Says federal judge Clark Waddoups in Brown v. Buhman, a case about the Utah anti-bigamy statute, which makes it a felony "when, knowing he has a husband or wife or knowing the other person has a husband or wife, the person purports to marry another person or cohabits with another person." From the first time I noticed this issue, I've thought the answer was obvious. You can't punish people for the ideas they happen to have about why they are living in a household with multiple sexual partners. Call it a marriage or call it a sandwich. Imagine that God blesses your relationship or imagine that your kitty cats brought you together. It's no proper concern of the government's.
Now, if you want to legally register your marriage and qualify for various marriage-connected benefits and privileges, it's a different matter, and Judge Waddoups makes that clear. This case was not about that. It was about people who live together and perform private marriage rituals and call themselves married. Prosecuting these people, while other married people are left alone when they commit adultery, is criminalizing their speech and beliefs. If you understand the issue, I think you'll see this must be the answer.
This does not lay the groundwork for finding a right to marry multiple spouses any more than it compels the government to prosecute adulterers.