books.google.com - "Enlightenment can only be realized through sexual union, the supreme Buddha activity."...https://books.google.com/books/about/Tantra_of_the_Tachikawa_Ryu.html?id=C4ftyxZCOOoC&utm_source=gb-gplus-shareTantra of the Tachikawa Ryu
John Stevens has written many books on Japan’s martial, sacred, and erotic ways—including the seminal study Lust for Enlightenment: Buddhism and Sex—and is a respected authority on Esoteric Japanese culture. He lived in Japan for thirty-five years, where he was a Professor of Buddhist Studies and an Aikido Instructor at Tohoku Fukushi University.
Tantra of the Tachikawa Ryu: Secret Sex Teachings of the Buddha