SYDNEY--In Japan, 13 same-sex couples will go to court on Valentine's Day calling for the right to marry, in a bid to become the first Asian country to allow gay marriage.

The couples will file suits on Feb. 14 in at least four cities, including Tokyo, claiming the government is violating their constitutional rights to equal treatment, said one of their lawyers, Yoshie Yokoyama.

"The constitution gives you the right to pursue happiness and equality before the law," Yokoyama told the Thomson Reuters Foundation in a phone interview from Tokyo.

"Not recognizing same sex marriage violates this."

Japan's laws on LGBT+ rights are relatively liberal compared with many Asian countries, with homosexual sex legal since 1880. But being openly gay remains largely taboo and opponents say such unions could destroy society and family institutions.

In a push to change attitudes, one Japanese lesbian couple plan to stage wedding photos in 25 countries where same-sex marriage is legal.

While gay marriage is illegal in Japan, in some areas couples can get a certificate that grants them similar rights to married couples, making it easier to rent apartments and visit each other in hospital.

Ai Nakajima is one of the plaintiffs. She and Kristina Baumann married in Germany last year but were unable to register their same-sex marriage in Yokohama city, near Tokyo.

This means Nakajima cannot apply for a visa for Baumann, who fears being deported if she fails to find work after her studies.

"There are many international Japanese same-sex couples, but since there is no law to recognize and protect our relationship, there are many people who choose to not live in Japan even if they want to," Nakajima said.

Same-sex couples in Japan are also denied parental rights to their partners' children and to inherit their partners' assets when they die, said Yokoyama.

Ging Cristobal, regional project coordinator for OutRight Action International, an LGBT+ advocacy group, said Japan could set a precedent in Asia.

"Japan can be one of the first Asian countries, like Thailand, Vietnam and Taiwan, to really show respect to equality and non-discrimination by guaranteeing rights are enjoyed by all citizens," she said in emailed comments.

Moves are under way in Thailand to debate allowing civil partnerships, while Taiwan voters rejected marriage equality in a referendum last year.

Vietnam allows gay marriage ceremonies but couples do not enjoy the same legal protections as heterosexuals.