In What Sense is Polygamy Currently Illegal?
The answer, I think, is that in most of the U.S, indeed most of the developed world, it isn't, even where there are laws that say it is. For the most part, restrictions on consensual non-marital sexual relationships between adults either do not exist or are not enforced. The same is true of restrictions on out of wedlock childbirth. So if three or more people want to engage in a long term sexual relationship, they are unlikely to be prosecuted for doing so.
What is illegal is marital fraud–engaging in what claims to be a legal marriage with one person without telling her that you are legally married to another. That, as I understand it, is the real substance of polygamy laws as they are currently enforced. In addition, of course, while a relationship involving more than two people may not be illegal, it also isn't legally marriage. That can matter in contexts such as inheritance, disputes over parental rights, and the like–the sort of contexts where same-sex marriage is still an issue even though same-sex relationships are legal. It also matters in the context of legal restrictions on sex by age, which frequently distinguish between marital and non-marital sex.
All of which seems to imply that the FLDS could legally conduct something close to their current marital practices if they were just a little more careful to conform to the letter, if not the spirit, of the relevant laws. To start with, they would want to locate in a state where the age of consent for marital sex is low--13 or 14--and not marry any girls younger than that age. They would have to refrain from having more than one wife younger than the age of consent for non-marital sex. A man who wanted to marry a second wife below that age would have to first legally divorce his first wife–but as long as she was at that point old enough to consent to non-marital sex he could continue living with her.
So far as I can tell, this would work, legally speaking. The remaining danger, assuming it were done openly, would come from non-legal objections to the practice which might take legal form. Texas raised its age of consent for marital sex from 14 to 16 only a few years ago. For all I know, that may have been a deliberate attempt to either get the FLDS in trouble or drive them out. And even with no legal changes, the law can be used selectively, as I think is happening in the present case, against a sufficiently unpopular minority.
It long ago occurred to me that there would be a simple way to maintain a low profile polygamous relationship in modern day America. Marry wife A, have a child by her. Divorce wife A, marry wife B, continue to live in the same neighborhood as wife A. If wife A is observed to be frequently in the house of husband and wife B–she would presumably want to maintain a legal residence somewhere else–that is easily explained by the desire of both parents to spend time with their child.
Does anyone know of counterexamples to my legal claim? Have there been people prosecuted for polygamy in recent years as a result of living and having sex with two or more people, where all of them were consenting adults?