Following a lawsuit filed by a married straight couple who claimed the right to be able to register their marriage under one surname, which is not possible according to the Civil Code, the Supreme Court which heard their case said that their request was not a violation of the Civil Code.
But, in their decision, the judges cited Article 24 of the Civil Code and this article matters since it is the one that defines marriage.
To understand well, you have to know that the Japanese do not read the text of Article 24 in the same way because of the word "ryōsei". This word can be translated in two ways. So, some consider this article defines marriage as a union between opposite sex couples, while others include this word as "two parties."
However, as the Supreme Court made its decision on Article 24, the Supreme Court may have favored the second translation, which defines marriage as a union between two people, regardless of gender precision.
Japanese Prime Minister Shinzo Abe has said the constitution has to be first revised to allow equal marriage.
But a precedent was created.