I was the one who inserted the information about Hiraga Gennai's homosexuality into his Wikipedia page. I've read some about him, and in my view no thorough biography of Gennai can miss the fact about his sexual orientation; Gennai was very concerned about homosexuality both as an intellectual matter and as a personal source of pleasure. He was open about his sexuality and, in fact, that was generally the case in Japan back then. There was little stigma against homosexuality, a fact recounted with shock by Christian missionaries who visited the country in the 16th and 17th centuries. Prior to the 19th century, when Western influence started to expand over Japan, it was thought to be normal for men to be attracted to both sexes, though it was known that a few exclusively liked only one. Gennai was one of them, and he only liked males. He was very present in the male quarters of red light districts of Tokyo (then known as Edo) and shunned the female quarters altogether. By Gennai's time, it seems male prostitution was already on the wane, which Gennai attributed to actions of the state against male prostitution. In his writings Gennai very much deplored this fact and blamed the perceived decay of the Japanese society of his times on the increasing hetero-eroticism of his contemporaries. Things would only get worse in the 19th century, but fortunately Gennai didn't come to witness that.

If you want some direct sourcing for his sexuality, here's an excerpt from this book (page 86):

He was also one of the most popular writers of the late Tokugawa period and a regular patron of the male brothels referred to as kodomo-ya. His friend, the leading scholar Ota Nanpo, recorded that "when [Gennai] had money, he hurried to spend it in Yoshicho on the pleasure-boys. There he would spend days at a time, and that is why he glorified nanshoku in Nenashigusa."

Nanshoku, which literally means "male colors", was a term for male homosexuality in Tokugawa Japan.

Another excerpt (page 102):

Gennai himself; according to Ota Nanpo, often visited Yoshicho and the "southern wards" of Edo, with their nanshoku teahouses, but never went to Yoshiwara, with its female prostitutes. His biographers claim the scholar was not simply a shudo-zuki but an onna-girai, or "woman-hater. Men identified as such, like similar characters in Ming-Qing or even in English Augustan literature, find any contact with the female sex revolting.

Shudo-zuki means "lover of lads".

I hope your doubts have been satisfied.