Marriage is great - and the more loving couples that want to sign up, the merrier we should be. With all three main party leaders and the majority of MPs backing same sex marriage, changing the law should be straightforward. But reactionary voices have been noisy and Number 10 has started to wobble.

We need to remind David Cameron to show a bit of backbone; the case for same sex marriage is extremely strong whilst the main complaints don't withstand scrutiny. We need to be loud and proud supporters of equal marriage and show the Prime Minister why he must stick to his pledge.

If two people love each other and want to make a long term commitment, that should be cause for celebration whatever their sex or sexuality.  When the Labour government introduced civil partnerships seven years ago, it broke new ground and was very controversial. Their celebration and the determination of campaigners mean more and more people are supporting change. Parliament should take the next step to eliminate the remaining discrimination in the way the state views loving relationships. To deny couples the recognition of marriage by the state is unjust and out of date.

Sadly the backlash against same sex marriage has started; no mention in the Queens Speech, not even a promised draft bill, and a stream of briefings about Number 10 u-turns and the doubts of senior Ministers.

Unconvincing

Yet the arguments against changing the law are deeply unconvincing.  Tory Ministers who argue it shouldn't be a priority and should be stopped for fear of distracting from the economy are missing the point. It isn't same-sex marriage that is undermining jobs and growth. George Osborne is doing that. And it isn't new legislation on love and commitment that has caused a double dip recession - the Government has achieved that all on its own.

Nor can anyone plausibly claim it will take up too much Parliamentary time. When the Civil Partnerships Bill was being passed in 2003/04, it took less than 1% of available Parliamentary time in the Commons chamber — 11 hours and 11 minutes out of a total of over 1,200 hours of possible debate. Especially with agreement between the Party leaders, there is no reason why this Bill should be bogged down.

As for the traditionalists' claim that same sex marriage undermines the very concept of marriage, it's clear they have a narrow view of history.

Marriage has never been a rigid and unchanging institution.  The cornerstone of marriage that most of us value today is the loving long term commitment two people make to each other.  But other traditions, obligations and requirements have changed and evolved across time and culture.

In ancient Greece, marriage ceremonies involved ritual sacrifice. In ancient Rome, concubines were often accepted alongside legal and legitimate wives. In ancient China, betrothal decisions could depend on fortune tellers reading birth charts. For centuries in Britain wives lost their separate legal identity and surrendered their property to their husbands. Even until the 1990s, wives in the UK were denied proper legal protection against rape by their husbands.  All that has changed - and rightly so.

Progress or prejudice

Marriage is more than just a historic tradition - it is a building block for a secure and stable society. Some claim extending marriage to same sex couples weakens the concept, but the reverse is true. It is only when marriage loses its relevance to communities, or is seen as outdated or unjust, that it risks becoming weakened or forgotten.

And we shouldn't be put off changing civil marriage by opposition from some church leaders. Nor should we forget that there are many different views within and between different faiths. No-one is proposing that churches be obliged to hold same sex marriages.  Freedom of religion is a very important part of our society. Religious marriages will continue - as they have always been - to be a matter for each church and denomination, and not for the government. But civil marriage - separate from the religious sacrament and stretching back 400 years - is rightly something the public and Parliament can choose to update.

Those religious organisations who want to support same sex marriage should be able to do so too. Currently the government's consultation rules that out. Yet respect for religious freedom means that we should respect the choice of Quakers, Unitarians and others to support same sex marriage.  I hope that, over time, even more faith groups will follow in their footsteps. When Parliament debates this, we will propose amendments to give religious organisations that choice.

For all the progress Britain has made over the last few decades, we should never forget that too many people still face prejudice. 45 years ago, homosexuality was still illegal.  In the 1980s and 1990s, schools were inhibited from tackling homophobic bullying by Section 28. And discrimination in goods and services was outlawed only recently. Time and again we have rightly changed the law to tackle outdated prejudice, discrimination or homophobia around employment, services and criminal justice. In the interests of justice, it is time for the State to show equal recognition and respect for same sex relationships too. David Cameron should stop wobbling and bring forward legislation now.