" Since marriage as an institution was adopted by various religions long before governments became involved with it as a legal formality it should be largely for them to decide whether they want their institution changed so radically. "
Unfortunately, it's not that simple given that the Church of England is part of the state...
There's also the fact that the current set of laws actually prohibits churches like Unitarians and some Anglicans who are completely in favour of gay marriage from carrying it out. I'd actually agree that no church should be forced to carry out gay marriages (what'd be the point?), but legalizing gay marriage is a necessary prerequisite to allowing the various churches to sort it out for themselves. Parliament would not have to 'waste its time' on the issue if the established set of laws were not so inadequate for allowing that to happen.
AOG:
"
But why must they demand to get 'married', a term used for the union between heterosexual couple for thousands of years. "
It's about participation - let me explain: Marriage, as you say, is a very old cultural institution which is highly valued in this country. It is so old, in fact, that it pre-dates a time when what are today called gay relationships were recognised as being valid (or even existing). All that gay people are currently asking is the ability to participate in an established cultural ritual precisely because they value it and, just like everyone else, they see it as important. Most married couples (including yourself, I'm assuming?) don't really have any objection to allowing gay couples the right to participate, and there are some faith organisations which are quite happy to perform it, so why not? If gay couples were out to destroy marriage as an institution, they wouldn't seek to participate in it.
As for the minority-majority argument, as was demonstrated to you earlier by sp, there is not much evidence that the majority are actually opposed to these measures - if anything, they seem to be in favour or at least acquiescent. Further, the reason these laws need to be changed is that they do put a minority on an unequal footing - gay couples who wish to get married, and faith organisations who wish to marry them are actually barred from doing so. Given that everyone seems to agree that there's no real reason why that should be so, why shouldn't the govt change it?
"Also why do the words Husband & Wife, Bride & Groom, Mr & Mrs, have to disappear, can't gays also hold these titles, instead of special descriptions being made to suit their requirements?"
You are talking about the presence of the words on forms - which is purely an administrative convenience. If you accept the argument I've outlined above, it doesn't make any sense to have a convoluted system particular forms for particular couples and instead just to adopt gender-neutral terminology. It's nothing more than an administrative convenience. This is not the same thing as the word disappearing from society, not by a long shot. And I think you know that full well.