"One of the puzzling questions is why long-term cohabiting couples bother to legalise their relationships," the study says.
"In New Zealand, these couples are considered by the state to be in a 'marriage-like relationship' after they share a residence for three years, or earlier if they reproduce together.
"This means that they are required to support each other financially, and separating couples are normally expected to divide equally any property accrued during cohabitation."
Legal marriage has declined dramatically. The proportion of couples living together in their 20s who were legally married dropped from just over half in 1996 to only a third just 10 years later.
Almost half (48 per cent) of all babies born last year, including 35 per cent of non-Maori babies and 79 per cent of Maori babies, were born to parents who were not legally married.
New Zealand's share of births outside marriage was one of the highest in the developed world, compared with 34 per cent in Australia and an OECD average in 2009 of 36 per cent.
Ten marriage celebrants interviewed for the study said that "all or the vast majority" of the couples they had married were already living together, and many already had children together.
One couple had lived together for 23 years and already had grandchildren when they finally married.
Yet the study found that few couples cohabit forever. "Most either eventually marry or separate."
By their 40s, 80 per cent of couples living together are legally married, and among over-60s, the figure is more than 90 per cent.
Forty individuals, including 13 gay people, who were asked about why they chose to marry or have civil unions after living together for at least three years, said they did it "to make a public commitment and celebrate a successful relationship".
Some felt pressured into it by their parents - and by their children. A middle-aged man who had been living with his partner for 15 years said their son kept asking his parents "every now and then" why they were not married, until they finally agreed to do it.
"I think most of his classmates' parents would be married," the father said.
Despite the modern ethos of equality, the study found that men were still expected to propose to women. "The proposal usually followed the patriarchal tradition and came from the man, while in same-sex couples it often came from the older partner," the authors found.
Almost all couples now give each other rings in their wedding ceremonies, but usually only women get an engagement ring first to symbolise that they are "taken".
"The engagement ring operates much more symbolically now. That is the one where it's gender-differentiated and remains so," Dr Elizabeth said.
"The diamond engagement ring was marketed to women as a symbol of romance, and of course it has a huge commercial impulse. They did try in about the 1930s to have a male engagement ring in the States, but they just couldn't get it to take off."
Most brides still choose white dresses. "Of course it has moved away from being a dress that marks your virginity to be an often very sexually alluring dress, but it's still seen to be quite pivotal to what a wedding is about," Dr Elizabeth said.
And most still walk up the aisle with their fathers. The study quotes various celebrants estimating 80 or 90 per cent of brides are still escorted in by their fathers, even when the "aisle" is in a garden or on a beach and even in same-sex civil unions.
"One of the lesbian couples talked about both being walked in through the guests in the garden to the podium by their fathers," Dr Elizabeth said. "A lot said they were happy to be walked up the aisle but not to be 'given away'. A lot of the women said, 'I want to honour and respect my father and recognise that this has been a longstanding part of the marital tradition, but we won't do the traditional notion of my father giving me away'."
Modern women also draw the line at promising to "obey" their husbands, Dr Elizabeth said. "Almost all the women said they wouldn't have said, 'I promise to obey'."
