With gay marriage back in the headlines, it's telling to look at the numbers and take stock of the campaign to stop marriage equality

Tomorrow MPs will vote on the second reading of Louisa Wall's marriage amendment bill. On its first reading the bill passed comfortably, 80-40. So seeing just how many MPs change their vote will be an interesting measure of how effective socially conservative groups are these days.

Those supporting the bill have every right to be confident; there's little doubt the bill will pass. There's hardly any sign of the very public anger that accompanied the Homosexual Law Reform Act in 1986, when Invercargill MP Norm Jones thought it acceptable to tell gays to, "'Go back into the sewers where you come from".

For an indicator of social change, look at John Banks. On the decriminalising of homosexuality in 1986 he said:

“This day will be remembered as a sad and sickening day for New Zealand”

This week he'll vote in favour of gay marriage.

Yet I fear there is more disquiet about than has been publicly acknowledged. Social mores are such that open bigotry is no longer acceptable and that, happily, is moderating the mood and language. But despite the assumptions of many I meet that this is a no-brainer, beyond the socially liberal fringes of the media and beltway I suspect a significant number of New Zealanders are at least uneasy with this bill.

Those opposed to the bill have found a line that's acceptable for the times – that 'New Zealand has civil unions which give gays and lesbians equality before the law. So why do they need more?'. The argument is that they've got a fair go now, why do they need to come around upsetting our marriage apple cart.

It allows someone to say they're not anti-gay per se, but just protective of the institution of marriage.

If that line had been latched onto sooner the bill may have had some difficulties, as it gives people to take a more conservative stance without expressing outright prejudice. And I think as the vote gets closer, public sentiment has become more wary of change, as it often does in votes on historic social change.

Look at what few polls have been taken on this. In June last year a One News-Colmar Brunton poll gave the bill 63 percent support; in December a Herald on Sunday poll had support at 54 percent. (There was a Curia poll this year for Family First this year, but the question was so leading it's hardly worth the mention).

A Research New Zealand poll in the middle of 2011 put support at 60 percent. In September last year it was 49 percent. It's worth noting that, if New Zealand First's view held sway and this went to a referendum, I'm not at all sure it would win.

It's easy to downplay this vote as natural evolution of a liberal society, and a done and dusted deal. But it is undoubtedly a major step as a society.

Look at the 23,000 public submissions to the select committee. Yes, many were form letters, but that doesn't mean a lack of belief.

And those opposed see this as a sign of something terribly wrong with modern society. They cannot understand how anyone can see marriage as anything other than between a man and a woman... that there is no right to marriage but only one bestowed by society... and that a vocal minority is embarking on a social experiment that could harm families.

The problem with their campaign, as far as I can see, is that while it seems the public mood has shifted, it's been insufficient to move the politicians. In the end, it's the MPs who count (and vote) and those running the campaign seem to have misjudged. Perhaps they should have targetted the politicians themselves more directly.

It's a sign of how far New Zealand – and indeed many western countries – have come in a generation that a majority have more flexible ideas about families and how they can function. As is often the case, personal experience has overcome social expectations.

But I remain interested to see how many MPs change their vote tomorrow. More than a handful said they voted for the first reading to have the issue debated. Most will feel happy at the lack of public angst and will feel safe to go ahead and vote yes. But will conservative campaigns have changed the minds of many?

I'll be curious to see the numbers tomorrow, but not surprised if the majority has shrunk just a bit.