A fraudulent woman was forced to pay compensation to a man for having sex with his wife.
The Tokyo District Court ordered the nameless 37-year-old woman to pay 110,000 yen (£ 725) at a hearing on February 16.
The rare court ruling recognized that illegitimate sex between same-sex partners is infidelity by law.
The 39-year-old husband filed a lawsuit against his 35-year-old wife’s lover in connection with the affair after the couple met online.
The defendant argued that her actions did not break the marriage and thus did not constitute infidelity.
But the court ruled that the affair had undermined the peace, reports Asahi.
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The ruling reflects the changed attitude towards same-sex adultery in Japanese law.
It comes after the Tokyo Supreme Court ordered a woman last March to pay compensation to her partner after cheating on her.
It recognized that same-sex union should be treated like marriage.
The couple had lived together for seven years, married in the United States and talked about having children together.
The judge ordered the defendant to pay the applicant damages of 1.1 million yen (£ 7,265.18) Pink messages.
“It was a relationship similar to where a man and woman come together to live together as a couple,” said Judge Hitomi Akiyoshi.
“A same-sex couple is an agreement between two people and on this basis it can be said that they have the same obligations of fidelity that a legally married couple of different sexes makes.”
Japan remains the only country within the G7 that does not legally recognize same-sex couples at all.
That month, the Sapporo District Court ruled that the government’s failure to recognize same-sex marriage was unconstitutional.