gekko you may not agree, but here's the issue that happens in sooooo many of the polygamous marriages (at least in Utah) ... the man cannot make enough money to support all the wives and children and so the wives apply for aid in its various forms from the state and claim they don't know who the father is ... or that they are not married (which they aren't by state code but they claim to be so anyway in all other arenas). So in a slightly indirect way, the state does have a very big interest in monogamous marriages being what is acceptable.

I happened to grow up in an area of the west where there were a lot of polygamists not too far away ... I know that in Utah when they prosecute polygamists it has never been for polygamy but about other things such as theft of services from the state as mentioned above, or in harmful treatment of family members who have appealed to the state for them to do so ... those who live in polygamy in the western US for the most part are not prosecuted in great part because they live in close communities or farming type complexes and they are unable to gather sufficient evidence to prove a case.