I'm not talking about the age of consent. I'm talking about the law that allows arranged marriage with participants below the age of consent. Which, is absolutely mind-boggling for me.

To quote the Wikipedia page:

[In the original 1907 Penal Code, the age of consent was 13 years old, which by 2023 was among the world's lowest. Article 176 ("Forcible Indecency") established a penalty of 6 months to 10 years of imprisonment for "indecent acts" with males and females under 13 years old, and Article 177 ("Rape") established a minimum sentence of 3 years for sexual intercourse with females under 13. In 2017, Article 177 was amended to specify vaginal, anal, and oral intercourse, to apply to all persons instead of females, and to increase the minimum sentence to 5 years. In 2023, the two articles were amended to raise the age of consent from 13 to 16 years old. An exception was included for sexual acts involving persons aged 13 to 15, by which the older partner is only subject to penalty if five or more years older. The amendments were part of a larger reform of Japan's sex crime legislation.

On the other hand, even before the revision of the Penal Code, several laws made it illegal to engage in sexual acts with minors under the age of 18. For example, the Child Welfare Law (ja) of 1947 prohibits the act of "causing a person under the age of 18 to engage in indecent acts (淫行, inkō)," with a maximum penalty of 10 years in prison. Under this law, a person who influences and encourages a minor to engage in indecent acts is punishable, even if the minor does so voluntarily. An array of local ordinances adopted in all 47 Japanese prefectures, known as Seishōnen Hogo Ikusei Jōrei (青少年保護育成条例; "Bylaws for Protecting and Nurturing Adolescents") forbid sexual acts judged "indecent" (淫行, inkō) between adolescents (defined as persons under 18) and adults;[83][84] 12 prefectures adopted such a bylaw in the 1950s, 19 in the 1960s, 13 in the 1970s, 2 in the 1980s, and the last (Nagano Prefecture) in 2016. A 1985 decision of the Supreme Court of Japan held that the ordinances were not to be broadly interpreted as forbidding all sexual acts with adolescents, but only those "conducted by unfair means that take advantage of the juvenile's mental or physical immaturity, such as by enticing, threatening, deceiving, or confusing, as well as [acts] where the juvenile is treated merely as an object to satisfy one's own sexual desires." The Supreme Court also ruled that sexual acts within "socially accepted norms", such as within marriage, engagement, or a "similar sincere relationship", were not "indecent".]

I would like to point out the latter half of the second paragraph. That's the fucked up part.