Last week, I got into an online debate with a fellow blogger about why urbanites are so obsessed with the Conservatives' position on gay marriage.

I'm aware that might be the least enticing lead sentence to a column you're going to read this year. But bear with me. I'm going somewhere with this.

The argument from Chris Selley - who runs www.tartcider.com - was that his youngish Toronto acquaintances were merely using the marriage issue as an excuse to justify their anti-Conservative leanings. As he put it: "These people are latching on to same-sex-marriage, to which they hadn't devoted a moment's thought until 2003 at the earliest, to avoid reconsideration of their ingrained Torontonian predilection to vote for the left-most centrist candidate, which in turn betrays a fear of change most unbecoming of people their age."

His acquaintances' reaction was more genuine than he was giving them credit for. With gays the last group - or at least one of the very few - against whom it's still widely considered socially acceptable to express prejudice, this is our generation's opportunity to extend rights to those who lack them. In that context, particularly if you live in a city with a large gay population, it feels awfully regressive to register a vote that may lead to those rights being reversed.

Three days later, the Conservatives were held to a slim minority government because they couldn't win a single seat in Toronto, Vancouver or Montreal. Whether same-sex marriage was a stumbling block or just a ready-made excuse, it clearly didn't help. If they're ever going to win a majority, the Tories still need to convince urbanites that social conservatism is not high on their agenda.

How to do so? Simple: To make clear that gay marriage isn't one of your top priorities, make it one of the first things your government tackles.

In opposition, Stephen Harper's rationale for vowing to reopen the issue was that not all MPs were allowed to vote their conscience in the last parliamentary vote. The logic is a bit sketchy, since other than New Democrats (virtually all of whom support gay marriage) the only MPs who were whipped were Liberal Cabinet ministers - and a full 13 of them would have had to vote the other way to change the 158-133 outcome. But at least there's a defensible case in there somewhere - particularly since this time it would be framed as a straight-up vote on gay marriage, rather than the phony rubber-stamping of a court decision that Paul Martin sold it as last time.

What would be indefensible, though, would be for Harper to borrow a page from Quebec separatists and wait until he had what Lucien Bouchard would call "winning conditions" - i.e. enough Conservative MPs in the House of Commons - to overturn gay marriage.

We've already had two election campaigns in which the issue was on the table and candidates' views on it were well known. The first time, Canadians elected more MPs in favour than against. This time, it appears, they've done so again. It would be an ugly tactic worthy of the Parti Quebecois for the Tories to try to wait until we finally stumble onto a parliament with more against than in favour, and then overturn the new law. And insofar as the Tories hope to win a majority, it would be political suicide - confirmation of the "hidden agenda" urbanites darkly speculate about.

There's another reason for Harper to hurry up with this. To this point, the Liberals haven't been able to fully exploit gay marriage as a wedge issue because Paul Martin was himself unwilling to make a proper case for it. The next Liberal leader probably won't be so squeamish. If the Conservatives wait even until later in this parliament to hold the vote, they'll be handing him a ready-made cause to wrap himself around.

In one fell swoop, Harper has a chance to appease his party's social conservative base by delivering his promised free-vote, and reassure social liberals by having it defeated. That's the only way for all of us to move on. Because if Selley's friends are still obsessing over gay marriage next election, the Conservatives will win about as many urban seats as they did this time.