A district court in Osaka, Japan has upheld the constitutionality of the nation's ban on same-sex marriage.

The plaintiffs in the case are two male gay couples and a lesbian couple.

The ruling, issued Monday, stated that marriage is defined as a heterosexual union, according to Reuters.

Japan is the only Group of Seven nation where gay and lesbian couples cannot legally marry.

Monday's ruling was only the second on marriage equality in Japan.

A ruling last year in the city of Sapporo declared Japan's marriage ban to be unconstitutional.

A lawyer for the couples said that they would appeal the ruling in Osaka.