For the couple, it will be a legal union culminating a relationship of more than five years.
For France, it will be a highly mediatised symbol of changing social mores, won in the teeth of months of fierce -- and sometimes violent -- opposition from conservative groups and homophobic backlash.
Vincent Autin, a 40-year-old gay activist and boss of a Montpellier PR firm, and Bruno, a 29-year-old government worker who did not wish to have his last name published, are more than conscious of the import of the upcoming ceremony.
"We will make this wedding an occasion for everyone. It will be public, open to all activists, to heads of French and international (gay lobby) groups, to the press," Vincent told AFP.
"There will also be moments of love," he added quickly, to reassure Bruno.
The two men said that, as significant as the wedding would be, it was just one step towards a bigger goal: to start a family by adopting a child.
"The law will allow that, but we're very aware that we won't have the child we both want right away. Mentalities have to change. And of course the path to adoption is long, even for heterosexuals," Vincent said.
Bruno, quieter than his partner, agreed. "Everything won't get done from one day to the next."
While 13 other countries have already changed laws to permit same-sex marriages, France's move to join them has run into determined protests.
On Sunday, a big demonstration of conservatives was held in Paris in a last-ditch bid to try to prevent the French parliament on Tuesday passing a bill allowing gays to marry and adopt children.
Other protests have taken place, including a huge rally of more than 300,000 in the capital in March, and running street skirmishes with police last week.
Lawmakers in parliament also nearly came to blows in a final debate of the bill.
But with the legal change being a key manifesto pledge by President Francois Hollande, the opposition looked certain to fail.
Montpellier's mayor, Helene Mandroux, from Hollande's Socialist Party, sees the shift to gay marriage as overdue -- and a boon to her town, which was recently named by French gay magazine Tetu as the most gay-friendly place in France.
Montpellier, situated on the Mediterranean coast and France's 8th-biggest city, is sometimes known as the "French San Francisco" because of its thriving homosexual community.
It was partly for that reason that government spokeswoman Najat Vallaud-Belkacem in September promised Mandroux that Montpellier would celebrate the country's first gay marriage.
The event is likely to take place at the end of June or the beginning of July, Mandroux said.
"Obviously, the opponents will be there. But maybe those who say no, and who have the right to say no, will understand that the (French) republic is more powerful than violence," she said.
Vincent and Bruno said they keenly felt the vehement protests in the run-up to the gay-marriage law as attacks on themselves.
"Our identity, our capacity to love, to raise children were being challenged," Vincent said.
He added that he considered the anti-gay protesters to be standing against French principles.
"We are the ones defending the values of the republic."
"And we are also fighting for the children of some of these opponents who, tomorrow, will discover they are homosexual."