The Valentine's Day lawsuits to be filed in Tokyo and in other courts around the country on Thursday argue that the law violates same-sex couples' constitutional rights to equality.

They want the government to follow the example of many other nations in guaranteeing marital freedom.

Ten Japanese municipalities have enacted "partnership" ordinances for same-sex couples to make it easier for them to rent apartments together, among other things, but they are not legally binding.

Ken Kozumi and Kenji Aiba, who have lived as a married couple for more than five years, are among the 13 couples filing lawsuits.

"Right now we are both in good health and able to work, but what if either of us has an accident or becomes ill? We are not allowed to be each other's guarantors for medical treatment, or to be each other's heir," Mr Kozumi, a 45-year-old office worker, said in a recent interview with his partner Mr Aiba, 40.

In a country where pressure for conformity is strong, many LGBT people hide their sexuality even from their families, fearing prejudice.

The obstacles are even higher for transgender people, who face extra difficulties in a highly gender-specific society.

Resistance to allowing full gender equality was evident in a Supreme Court ruling last month upholding a law that effectively requires transgender people to be sterilised before they can have their gender changed on official documents.

"Many people don't even think of a possibility that their neighbours, colleagues or classmates may be sexual minorities," said Mizuho Fukushima, a lawyer-turned-lawmaker and an expert on gender and human rights issues.

"The pressure to follow a conservative family model, in which heterosexual couples are supposed to marry and have children, is still strong."

Prime minister Shinzo Abe and his ultra-conservative supporters have campaigned to restore a paternalistic society based on heterosexual marriages.

The government has restarted moral education class at schools to teach children family values and good deeds.

"Whether to allow same-sex marriage is an issue that affects the foundation of how families should be in Japan, which requires an extremely careful examination," Mr Abe said in a 2018 statement.

But while the law and many politicians lag behind, public acceptance of sexual diversity and same-sex marriage has grown in Japan.

According to an October 2018 survey, more than 70% of the 6,229 respondents aged 20-59 said they support legalising same-sex marriage.

Australian Associated Press