The UK's highest court is to rule on the case of a heterosexual couple who want the right to have a civil partnership.
Rebecca Steinfeld, 37, and Charles Keidan, 41, want a legal union through that route but are prevented because the Civil Partnership Act 2004 says only same-sex couples are eligible.
The academics, who live in Hammersmith, west London, suffered defeat at the Court of Appeal in February last year, but were given the go-ahead in August for a Supreme Court hearing.
A panel of five Supreme Court justices, including the court's president Lady Hale, heard the couple's case in May and will announce their decision on Wednesday.
The couple, who have two daughters aged two and nine months, claim the Government's position is "incompatible with equality law".
During the hearing, their barrister Karon Monaghan QC told the court they have "deep-rooted and genuine ideological objections to marriage" and were "not alone" in their views.
She said matrimony was "historically heteronormative and patriarchal" and the couple's objections were "not frivolous".
Ms Monaghan added: "These are important issues, no small matters, and they are serious for my clients because they cannot marry conformable with their conscience and that should weigh very heavily indeed."
The Court of Appeal agreed the couple had established a potential violation of Article 14 of the European Convention on Human Rights, which relates to discrimination, taken with Article 8, which refers to respect for private and family life.
But, by a majority of two to one, the judges said the interference was justified by the Government's policy of "wait and evaluate".
The Government said it was decided, after public consultations and debate in Parliament, not to extend civil partnerships to opposite-sex couples, abolish them or phase them out at that stage.
The aim was to see how extending marriage to same-sex couples impacted on civil partnerships before making a final decision which, if reversed in a few years' time, would be disruptive, unnecessary and extremely expensive.
In a statement outside court ahead of the May hearing, Ms Steinfeld and Mr Keidan said the Government should "stop making excuses" and act now to allow civil partnerships for all.
The court is due to deliver its ruling at 9.45am.