The opening skirmishes were won by supporters of gay marriage, but now we're into trench warfare in the Public Bill Committee considering the detail of the Marriage (Same Sex Couples) Bill.
Important things can happen in these committees, but they don't get too much direct coverage in the media because most of their proceedings is unintelligible to anyone who doesn't have a pile of paperwork before them. Votes on "clause stand part" motions and insert extra words into particular clauses are pretty difficult for anyone not intimately involved to follow.
But Public Bill Committees (they used, misleadingly, to be called Standing Committees) are now allowed to call in witnesses - and the evidence given across two intensive days this week was sometimes quite riveting - both for content and for drama.
There was an early spat between the Education Secretary, Michael Gove, and the Conservative committee member Tim Loughton, over whether teachers might find themselves in hot water if they were opposed to gay marriages and said so, perhaps during sex education classes.
Mr Gove thought it was unlikely a teacher would set out to do so; Mr Loughton thought it distinctly possible, and neither seemed very impressed with the other's point of view. And given the backstory: that Mr Loughton was sacked as one of Mr Gove's junior ministers, last September, and that he has had a couple of sharp clashes with Mr Gove's department since, there was a drop of real venom in their exchanges.
Then there were the superlawyers. Lord David Pannick and Lady Helena Kennedy both thought the protections in the bill for churches and individuals who had conscientious objections to gay marriages were watertight.
Lord Pannick argued that there was no danger of a successful challenge under human rights law, because the European Court of Human Rights precedent was firmly against the idea that people had a human right to a particular kind of religious marriage - so if a marriage was refused for religious reasons, the courts would uphold that decision. He quipped that there was more chance of the Red Sea parting than of the ECHR changing its mind. DUP MP and opponent of the bill, Jim Shannon, shot back that he believed in miracles, and Lady Kennedy retorted that miracles were rare in the courts, and when they happened they tended to involve juries, rather than judges.
Supporters of the bill were wowed by their evidence; opponents noted that other leading lawyers disagreed, and remained unconvinced.
Then there was the clash between the bill's supporters on the committee and some of the witnesses who were opposed. Labour's Siobhain McDonagh delivered a withering rebuke to the Roman Catholic Archbishop of Southwark, Peter Smith, when he remarked that some people might leave the church over its opposition to same sex marriage - saying she remained a devout Catholic, despite disagreeing with the church's teachings on a range of sexual issues and she would not be leaving. On the implications for teachers, she accused him of resorting to legalese, to show the Church in an unkind and uncaring way.
Then former Cabinet Minister Ben Bradshaw had a go at Colin Hart of the "anti" campaign, the Coalition for Marriage, saying he's opposed every extension of equality rights to gay people, adding his campaign was "about homophobia, pure and simple".
When I spoke to him after the committee sitting he conceded there was an element of payback: "Clearly if you are gay or lesbian and you have been subjected to the sort of hate and bigotry that's been coming out over many, many years over a whole host of issues, then it's rather tempting and welcome to, at last, have a chance to put those people on the spot, which we don't usually get."
The committee reconvenes on Tuesday 26 February, to start its detailed clause by clause debates. Logically enough, they start with clause one, which sets out to legalise same sex marriage, and opponents, led by the Conservative backbencher David Burrowes, will put down amendments aimed at getting the state out of the marriage business and bringing in a system similar to that in France, where the state registers partnerships for legal purposes, but where individual religions can decide who is entitled to a wedding under their auspices.
So what could be called a marriage was defined by religious organisations, which might answer the question in different ways, while the state didn't attempt to intervene in the definition of marriage.
To some this is beginning to look like embarrassing dad dancing on the head of a pin; to others it's a vital social, religious and ethical issue, and they'll fight it to the end. And in coming weeks, the battle will be fought out with passion and tactical cunning. I'm planning to blog on as much as possible of the action.
* You can see all the evidence sessions from the Public Bill Committee over on BBC Parliament, where they'll be broadcast one by one, from 6pm on Monday 18 February to Thursday 21. And I have a report on Today in Parliament on BBC Radio 4 at 11.30pm on Friday 15. (Available thereafter as a podcast, via the Radio 4 website).
** With Parliament on its half-term break, my regular blog on what's coming up in the chambers and on the committee corridor, won't appear till late next week.