Ok, I want to get this really really straight in my head, if you'll pardon the pun....
What is the problem with allowing Gay Marraige over there? If it is a travesty in the 'eyes of God' then, that shouldn't be a problem, they've been that (according to Christians) long before they got married.
Who said Marriage was a purely Christian tradition anyway? As the US found out to it's detriment, Moslims have them too

So, we are talking about people who obviously don't believe in God (if only for the fact that 'God' doesn't believe in them) or have chosen their own personal path. Since, like all marriages, it most certainly doesn't matter what religion the people in the street outside believe in.
And in fact, in that respect Gay's have got it right, look at the Lord's Prayer in the Bible, when his disciples ask Jesus how to pray, he replies...
But when ye pray, use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking.
8 Be not ye therefore like unto them: for your Father knoweth what things ye have need of, before ye ask him.
knoweth Matt 6:32, Luke 12:30, Phil 4:19
In other words, sitting in Church and droning out the 'Lords Prayer' like the Borg is the incorrect way to worship the Christian God

Your prayers should be your own, and no Priest should be leading people in prayer.
Anyway, if religion is not an obstacle, which it isn't, what scares people so much about it? It's not as if they are going to start shagging in your front garden once they are married, believe me, marraige, if anything, reduces the nookie count
