"Transforming the Fire of Lust"
The term "tantra" can create more confusion than clarity. First, there exist both a celibate tantric tradition known as "the right-hand path" and a sexual tantra tradition known as the "left-hand path." Second, there exist both a Tibetan Buddhist tantra and the classic Hindu tantra. Finally, even within the category of classic tantra, streams diverge...and have shifted with time. Classic sexual tantra is often not about mutual enlightenment. Despite the showy display of worshiping the divine god or goddess in man or woman, the energy exchange is for the purpose of solo spiritual attainments.
Frequently missing is the concept that glad giving leads to mutual transcendence. As a result, yogis not infrequently exude a fear of women or exploit them, which attitudes may account for the frightening image of Kali the goddess of tantra, with her necklace of skulls and lethal knife. Both fear and exploitation suggest that such yogis haven't overcome the separation between the sexes, which the Gnostic Gospels, for example, say anchors us to the illusion of the material plane.
Part of the confusion about the definition of tantra may stem from its meandering history. According to James N. Powell,  tantra's roots lie in a sex-positive, matriarchal society. One of India's earliest scriptures explains that the universe arose from copulation.
In the beginning there was only the Supreme Being, existing all alone....He had no delight. Therefore, he sired another. He divided himself in two, like a man and woman making love. From this division arose husband and wife (and ultimately all gendered species as the female hid herself in different forms and the male pursued her). 
A favorite divinity of these early worshippers was the lusty Krishna - a dark blue fellow who was a sort of sexual Pied Piper, luring otherwise well behaved married women from their beds to orgies in the wild.
Some worshipers of Krishna formed couples. They would stimulate intense, even violent, erotic emotions in themselves by reading and chanting the amorous adventures of their God. They would then perform sexual yoga, in which the male would play the part of Krishna and his partner the role of one of the cowherd girls [of the Krishna myth]. Love rituals were performed in large circles. It was felt that more intense erotic emotion could be produced if the women in such rituals were the wives of other men.
This heady combination of violent passion and adultery was echoed in the Cortezia, or courtly love, tradition - according to scholar Denis de Rougemont. Yet, classic tantra was not about raging passion. According to Powell, classic tantra emphasizes:
the pursuit and prolongation of preorgasmic pleasure rather than the quick production of orgasm. Assuming a number of poses in a single session has the effect of directing erotic sensation away from the genitals, lessening genital tension and thus the demand for orgasm. In this way every area of the body was bathed in highly charged erotic energy....Thus sex could serve as a form of yoga leading to divine communion.
Things changed as the paternalistic Brahmins gained control of India. Women were secluded, and tantra mutated. The Brahmins' divinity, Shiva, was less concerned with worship of the Divine Mother and more concerned with "the serene upward flow of semen."
Luminosity is the semen's natural and original state, but when passion arises it becomes "dark," building lustily like storm clouds where before it had been flaming upward luminously. Finally, at the moment of orgasm, it pours downward and outward.
Instead of mere sexual bliss, the reversal of the sexual energy could produce spiritual rapture and longevity. Semen came to be associated with solar light and was identified with the immortal spirit within the human heart. Shiva was said to have burnt up the God of Erotic Love with his inner fire fanned by avoiding ejaculation. The sacred text Siva Samhita says:
The falling of seed leads towards death, the keeping of one's seed is life. Hence with all his power should a [human] hold his seed. . . . In this world all is born from seed and dies of seed. Knowing this the yogi should always keep his seed. . . . By this practice even the [human] who indulges in worldly pleasures can attain realization [enlightenment], and all wished-for results can in this world be achieved.
Alas, as men's health and spiritual power were believed to depend more upon keeping their seed than upon sacred union, women were viewed with increasing suspicion. When Buddha renounced the world, he began with depreciating women. Scriptures say that he suddenly saw his harem (through a post-passion hangover?), as "a lake of lotuses broken by the wind." Women were thus increasingly the problem rather than part of the solution, and celibacy was glorified...except for those who desired a "fast path." Instead of seeking mutual enlightenment with a partner, the yogi now sought to awaken his own Feminine Principle within - residing in the form of the kundalini energy coiled at the base of the spine. Its release ideally manifests as psychological androgyny.
The most powerful method of arousing the kundalini was said to be the sexual embrace - preferably with a young virgin. Virgins were believed to be full of spiritual power and capable of initiating one into the flow of subtle energies. (Similar beliefs grew popular in China around the same period.) Yet, as Powell remarks about classic tantra,
This is certainly not romantic love....Here the female is appreciated not as a woman but as the Goddess, and it doesn't matter greatly if she is one's wife or a total stranger.
Techniques of breathing, chanting, muscle contractions, and meditation were used to transcend the human state and awaken the mystical energy that lies dormant.
The process is a contemplative, motionless surrender in which the sexual energy is allowed to move of its own accord without the interference of the will and gross physical stimulation. In this embrace increasingly subtle fields of energy unfold their soft textures and the union may thus deepen and expand for hours. The male retains his semen without much overt control, and both partners are left in a state of deep repose and fulfillment.
While classic tantra - even today - recommends techniques for transforming, or moving beyond passion ("vairagya"), Western teachers frequently adapt its methods to increase the passionate intensity of conventional sex. This is risky as there is significant evidence that increasing the intensity of passion harms us by raising the levels of dopamine in the primitive brain (and throwing us into an addictive high/low cycle).
Georg Feuerstein, founder of the Yoga Research and Education Center, has written a very thoughtful article that contrasts the New Age's "neo-tantrism" with the principles of traditional tantra with its "belief that sexual energy is an important reservoir of energy that should be used wisely to boost the spiritual process rather than block it through orgasmic release." Neo-tantrism runs the risk of bringing harm to its practitioners because it focuses on gratification of the ego's desires, distracting lovers from the spiritual goal of transcending illusion. According to Feuerstein, this risk is nowhere more apparent than in neo-tantrism's pro-orgasm stance.
At orgasm, "the creative tension that could serve as a bridge to ecstasy is lost. The whole point of avoiding orgasm is to accumulate the subtle force or nervous energy called ojas, which is wasted the moment the nerves fire during ejaculation." And it's not semen loss that's the problem. "It's actually the firing of the nervous system during sexual stimulation. That applies to both men and women," as Feuerstein explained in "Yoga Journal" in November, 2002.
Feuerstein differentiates the goal of traditional tantra, "bliss," in which the ego is transcended, with neo-tantrism's search for heightened, pleasurable sensations, which cannot lead to spiritual fulfillment. "Genital orgasms or whole-body orgasms are psychosomatic phenomena, not spiritual manifestations. Bliss is the everlasting "orgasm" of God and Goddess in divine embrace, beyond all concepts."
Importance of unconditional love
Author Robert Svoboda would say that the key obstacle to be overcome is "lust." To move beyond "donkeyhood," that is, eating and procreating, avoid lust. "When thoughts of sex fill the mind the Kama Agni (Fire of Lust) becomes inflamed. [Dopamine shoots up.] Heat is anathema to ojas. The Kama Agni causes a disturbance in the ojas, which alters the brain chemistry...." By contrast, harmonious thoughts increase ojas. For example, the unconditional love of motherhood actually increases ojas. Therefore, Svoboda advises men to see their partners as the Divine Mother, and women to see their see their partners as sons. In this way they can more easily stay in a giving mindset. That is critical to using tantra as a spiritual path according to Svoboda.
All desire for personal enjoyment or self-gratification has to be effaced from the mind. If you have even the slightest intention to take or gain something from your partner, that tiny desire can snowball when your nervous system is under full load, and an avalanche of energy can result, ending in [lust]. You must forget how to take and learn only how to give. 
Thanks to recent neurochemical findings, we now know that this ancient prescription for selfless love between partners is also a potent way to increase production of oxytocin. Oxytocin, the neurochemical behind both the parent/infant bond and the desire for monogamy, also aids healing, increases sexual receptivity, counteracts stress, and reduces cravings and impulsiveness. In short, tantra's true potential may hinge on transforming the "fire of lust" with unconditional love. Whether or not one admires classic tantra, it offers further evidence of hidden potential in intimate relationships.
- Energy and Eros, by James N. Powell, Lloyd, William Morrow and Company, NY (1985), p. 103, et seq.
- Love in the Western World, by Denis de Rougemont, Doubleday Anchor Books, NY (1957), pp 114-122.
- Excerpts from the Hindu text Siva Samhita.
- Visit the website "Transformation of Sexual Energy" of teacher Swami Satyananda Saraswati.
- "Traditional Tantra and Contemporary Neo-Tantrism" by Georg Feuerstein.
- Aghora: At the Left Hand of God, by Robert Svoboda, Brotherhood of Life (1986).