Living in AZ for over 20 years, I've had several Mormom friends & neighbors. They are good people. There are many denominations and some of them are in plural marriages. While that's not my cup of tea, it works for them. I don't see why they can't legally marry according to their beliefs. The state may wish to limit how many people in the marriage they recognize as being married- but they should be able to marry as many as they wish. Likewise, gay people have churches that do not believe that homosexuality is a sin, & religious leaders who wish to marry them. Yet, the government favors one set of religious beliefs over the other? It has no secular reason to support these laws. No economic rationale to support them.
The government's recognition of marriage should be limited to it's secular & economic interests. It should not favor one religious belief over another. The government's role is to establish an environment in which all different kinds of people, with different religious beliefs, can live freely and peacefully amongst each other, without favoring one religious belief over another.
"The establishment clause has generally been interpreted to prohibit 1) the establishment of a national religion by Congress, or 2) the preference of one religion over another or the support of a religious idea with no identifiable secular purpose." http://www.uslaw.com/us_law_dictionary/e/Establishment+Clause
The government has no secular or economic interests regarding marriage beyond limiting the number of people involved that it recognizes. The only reasons left for these laws are religious.
Yet, we have laws against polygamy and homosexual marriages. So much for "rendering unto Caesar..."