The heavy association with sexual practice seems to be a western preoccupation with Tantra. In most Buddhist studies, there is no mention of sex, and not even the sublimation of pleasure until you reach quite complicated areas of Buddhist theology.

Buddhist Tantra (as distinct from Indian tantra) is a largely Tibetan practice. The word Tantra refers to a class of Buddhist teachings, supposedly uttered by the buddha in various incarnations, and other boddhisatvas like Manjushri. Tantra claims to transcend the boundaries of Hinayana ("Lesser Vehicle", a feature of which was that only monks acheived nirvana), and Mahayana ("Greater Vehicle", in which all could attain nirvana, like Zen), and is consequently sometimes called Vajrayana, the Adamantine, or Indestructible Vehicle.

All Buddhist teachings aim towards the perfect awakening of a Buddha, and Tantra is no different.

The western association with sex seems to come from the Tantra attitude towards sensory perception. In all Buddhism, one needs to distinguish that the world of sensory perception is illusory. In Mahayana Buddhism, that is attained by denial ("There is no spoon"), or through ignoring them. In Vajrayana, the attitude is that the possession of sensory objects can also lead through enlightenment, through the skillful practice of Tantra.

It is stressed that the attainment of nirvana through this means is for the "skillful" and for people with "superior faculties". To do so otherwise, would be to immerse yourself more in sensory perception, and into samsara, the exact opposite of the goal of Buddhism.

Indian Tantra differs in its emphasis on Yogic Tantra, and the rejection of monastic celibacy (all for the good of course). Indian Tantra practice is also shamanic, in that there exists a body of spells for different purposes, including some from texts like the Hevajra. The ones I have heard of include love spells, and defeating enemies. Don't sound too pious to me.