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Mr Key said on Radio Live he would use a conscience vote to vote in favour of a bill allowing gay and transgender couples to get married.
He signalled his support for the bill in its early stages, but said he was unlikely to change his mind over the course of the debate to allow same-sex couples to marry - given it was a conscience vote.
"You go through all the merits of the argument and look at what people put up; but my view is that if two gay people want to get married I can't see why it would undermine my marriage with Bronagh," he told Radio Live.
"There will be plenty of people in our caucus who will be deeply opposed - particularly the very religious ones, and I can understand that," he said.
"I think it's quite healthy that New Zealand has the debate, I suspect it won't be what it was when you looked at homosexual law reform in the'70s - I don't think it will be that dramatic."
Mr Key said the bill was likely to reach select committee stage and he thought there would be a lot of lobbying from those opposed and those in support of the bill.
Opponents had mobilised against the bill allowing gay and trans-gender couples to marry.
Conservative Party leader Colin Craig said he was planning a campaign on the advantages of heterosexual relationships and traditional family structures.
"I am keen to be part of a campaign to get out there on the issue. It would look intelligently at the differences between homosexual parenting and a Mum and a Dad," he said.
Mr Craig, whose party received 2.65 percent of the vote in the general election, had committed part of his personal wealth to battling social policy in the past, such as Sue Bradford's anti-smacking bill.
- Kate Shuttleworth of APNZ