THE honeymoon hadn't even begun for newlyweds Narell Majik and Ash Watson before the High Court of Australia declared their marriage constitutionally invalid.

The couple tied the knot in Canberra just a day before the court struck down the ACT's same-sex marriage laws in a brief but blunt judgment on Thursday.

"It does not feel any different to how it did before," Ms Watson said after the ruling.

It was a sentiment shared outside the court by many gay newlyweds, who shared the forecourt with a group of Christian activists singing songs and waving signs decrying same-sex unions.

The outcome was widely expected, but the disappointment was still palpable even as couples proudly showed off their rings and the marriage certificates obtained under ACT law.

The first legal gay marriage on Australian soil took place less than a week ago.

Western Australia state upper house MP Stephen Dawson, who married his partner in the early hours of Saturday in Canberra, said the court decision was disappointing.

"The fight for recognition goes on," he tweeted.

For others there is a silver lining.

In their unanimous ruling, the five High Court judges concluded that only federal law could determine what constitutes a legal marriage, state or territory parliaments can not.

Australian Marriage Equality national director Rodney Croome said this clarified for the first time that federal parliament had the power to enact the laws for gay marriage.

"That puts the ball firmly in the federal government's court and increases pressure in particular on (Prime Minister) Tony Abbott and the federal coalition to allow a conscience vote on marriage equality so this issue can move forward," he said.

Others disagreed, saying the court's ruling had effectively put the issue to rest.

Australian Christian Lobby managing director Lyle Shelton said there had been 10 attempts in the past three years at state and federal levels to legislate a new definition of marriage.

"I think it's been given a fair go. I think it's time to move on," he said.

Mr Shelton expressed concern for same-sex couples who thought they were married under the ACT law.

"Understandably they will be disappointed at the decision handed down today and it is unfortunate they were put in this position," he said.

Attorney-General George Brandis said the government was pleased with the decision.

It was important for people to respect the High Court's authority, he said.

"Even if the decision had gone the other way ... it's imperative that all members of parliament, irrespective of their personal opinions, uphold and respect ... decisions of the court," Senator Brandis said.

Shadow attorney-general Mark Dreyfus said the decision confirmed Labor's position that parliament was the appropriate place for debate on same-sex marriage.

"The prime minister needs to give his MPs a conscience vote on this issue so that the national parliament can decide," he said.

Mr Croome said it was too early to tell whether the High Court decision left room for the ACT government to legislate in a different form.

But for Canberra resident Ivan Hinton, who married his partner Chris Teoh this week, the setback in the High Court hadn't dampened his determination to see his union validated.

"We will achieve marriage equality," he said.