Ultimately one person has to conduct the ceremony though.
Currently churches aren't forces to marry anyone - indeed many will refuse to marry unless they were fully convinced that both partners would keep their commitments. I know of two such churches with no or one divorce in 30 years. Do you want to force them to marry anyone who turns up?
This legislation seems to make a reasonable compromise by deliberately allowing religious institutions to opt out, so I find it ridiculous that they're kicking up such a fuss. If they're worried about the ECHR changing things, they should complain to Strasbourg.
I have to say, my personal preference would be for a Civil Partnership. The word "marriage" has too many connotations of religion for my taste.