Today I was in Court in Chorley Lancashire just down the road from one of the 2 Mormon Temples in Britain. The Mormon Church in Lancashire is currently running a Pageant at the Temple to celebrate 175 years since the first Mormon Missionaries arrived in Britain with many Mormons in America, including Mitt Romney, able to trace their ancestry back to those early converts. I confess I have a soft spot for the Mormons and have visited, though not entered (see Gallagher (Valuation Officer) V Church of Jesus Christ of Latter-Day Saints  UKHL 56 ) the Salt Lake City Temple and both British Temples. Mormon History is colourfull and fascinating.
So how you may be wondering does this relate to the passing of the Marriage (Same Sex Couples) Act 2013 ? The Act changes the definition of Marriage as historically understood in Britain by section 1(1) which says
"Marriage of same sex couples is lawful."
The previous legal definition of Marriage was first formally stated by Lord Penzance in the 1866 case of Hyde v. Hyde [L.R.] 1 P. & D. 130 where he said that Marriage in English law meant
" the voluntary union for life of one man and one woman, to the exclusion of all others"
A similar approach was taken by the US Supreme Court in the 1878 case of Reynolds v USA 98 U.S. 145
The common factor in both cases was that they concerned Mormon Marriages. As is fairly well known the Mormon Church practiced Polygamy (often called plural marriage) until it banned the practice in 1890 . In fairness to the modern day Mormon Church (proper title Church of Jesus Christ of Latter Day Saints) I should make it clear that it has never endorsed Polygamy since 1890 and the practice is totally forbidden by the Church. Nevertheless Polygamy was a major feature of 19th Century Mormonism and it is entirely because of that history that there was felt to be any need to formally define Marriage in Law.
So does the ending of the historical definition of Marriage open the way to Polygamy ? As is being discovered in Canada permitting Same-Sex Marriage makes it more difficult to put forward a logical argument against future attempts to change the law so as to permit Polygamy or Plural Marriage between consenting Adults.
However the change in the law which is happening on both sides of the Atlantic also raises the question of how far Government will go to enforce obedience to the new definition and there the 19th Century Mormon experience is instructive and sobering.
The practice of Polygamy by the Mormon Church was objected to by other Americans simply because they disagreed with it. Despite many lurid novels suggesting coercive Polygamy there is little objective evidence to support this and much evidence that many Mormon wives supported "the principle" as it became known and indeed spoke in its defence. Therefore provided the practice was voluntary one would have thought that the Mormons would have been left alone but that was certainly not the case.
The United States Congress passed a number of increasingly draconian laws against Polygamy and against Polygamists, ie Mormons, in ways that restricted their Civil and Constitutional rights way beyond simply banning the practice of Polygamy itself.
The 1862 Morrill Anti-Bigamy Act 1862 banned bigamy and limited church and non-profit ownership in any territory of the United States to $50,000 so targeting the Mormon Church in the Utah territory
The 1874 Poland Act eliminated Territorial Courts and Law enforcement officers in the Utah Territory, contrary to the normal practice of the US in its territories, and also restricted the ability of those living as polygamists to serve as jurors even if they had not been convicted of any crime. As was discussed in Reynolds v USA 1878 this provision led to a wonderful exchange with one potential juror
"Are you living in polygamy?" .... I am only a fornicator."
The 1882 Edmunds Act prohibited "bigamous" or "unlawful cohabitation" without requiring any evidence that there had been any form of Marriage ceremony polygamous or otherwise. It revoked polygamists' right to vote, made them ineligible for jury service, and prohibited them from holding political office.
Finally the 1887 Edmunds- Tucker Act dissolved
the Mormon Church and directed the confiscation by the federal government of all church properties. The legality and constitutionality of this extraordinarily draconian act being endorsed the US Supreme Court in Late Corporation of the Church of Jesus Christ of Latter-Day Saints v. United States 136 U.S. 1 (1890).
Fortunately for Mormonism the 1890 decision to abandon Polygamy allowed the Church to return to legality, eventually its property was restored and Utah entered the United States as a State with a Constitution which banned Polygamy.
The history of Mormon Polygamy might seem only of academic interest but I suggest that studying what happened is relevant today. What the Mormon experience shows is that ultimately Government is willing to use coercive force to defend its own definition of Marriage and to destroy those religions or forces that oppose that definition. The history of Mormonism also shows that Constitutional guarantees of Religious Freedom can easily be overridden and ignored in this situation.
Now that Government on both sides of the Atlantic are redefining marriage will they ultimately decide to convict those religious leaders who speak out against the new definition and abolish or expropriate the property of those religions that do not conform to the new ideology. Based on the Mormon experience I would not be surprised. As the Mormons showed changing the definition of something as fundamental as Marriage is a step with enormous consequences.