The outlawing of Polygamy is not favoring one religion over another. Marriage laws are secular, there is no religious component. Yes, many people get married in a church...but legally they do not have to. This is why same sex marriage is now legal in this country...because there was no secular reason to prohibit it. There IS a secular reason to prohibit polygamy...but a person can legally marry one person and then "religiously" marry a second person and there is nothing the state can do. As long as the second (or third or fourth) spouse is not claiming any legal benefits then it's all good.