Tantric Buddhism began as a revolutionary movement against the rigidity of traditional Buddhist monastic institutions.
What is Engligtenment interview:
" The founders of tantra came from all walks of life. We find royalty and aristocracy as well as tribal people and people practicing all kinds of trades and crafts. But interestingly, we also find people from the monasteries. As tantra was being founded and shaped, some of those in the monasteries left because they didn't want to be removed from life-as-lived. The main impetus for the movement, though, did take place outside the monasteries, from what we would call laypeople—people who wanted to practice yoga and spiritual disciplines, but not necessarily in a monastic context as celibates, and not in separation from members of the opposite sex or outside of the context of their intimate and familial relationships....
(the key practices of the tantric approach)
The basic mindfulness techniques and ethical teachings of Buddhism were already in place by this time. What was added at this point was the incorporation of a number of yogic techniques, specific ways of directing the breath and the inner energies of the body, which were drawn from the broader yogic knowledge of India. A lot of ritual elements were also incorporated, as well as magical techniques and dance practices. Probably what was most distinctive about this period, though, was the introduction of the yoga of union—the practices that men and women could do together in order to transform the energies awakened by sexual union into very refined states of consciousness, wisdom and bliss......"
Miranda Shaw, Ph.D. in Buddhist Studies, Harvard University, Assistant Professor of Buddhist Studies in the Department of Religion at the University of Richmond, author of Passionate Enlightenment: Women in Tantric Buddhism