Haven't watched the programme but as Mormon marriages aren't legally recognised in the UK, how could they possibly legislate against them? In the eyes of the law they are an unenforceable agreement the same as the many couples (or more) who have a bond of allegiance.
Legal marriage is purely a contract, the rest of the faff and expense that goes with it is profiteering by certain trades, much the same as Christmas, Easter etc.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates