Sex and Tantra

Sex
The symbolism of sex is used in Hindu and Buddhist imagery to show inter-relatedness. Rarely does it "refer" to actual contact. In Buddhism, where images are used as visual aids, a male-female pair stands for Compassion and Wisdom, and the pairing of knowledge with "skillful means" or, method.
Yab-Yum at the Holy Mountain site.
As an aspect of Hinduism, tantrism can include maithuna or sexual connection that usually has as its ultimate goal, the union of the individual soul with the Universal. Ecstasy of a sexual nature is only one of the lesser objectives, but that is how it has been most often misunderstood until recently. Sexual yoga can also be practiced with the objective of acquiring certain impressive abilities, accomplishments or siddhis. However, enhanced sexual experience is not the objective of any genuine yogic system or tantric tradition. That would go against the very foundation of a view that sees all beings as part of a greater whole that encompasses several different realms of existence; in which beings are dependant on each other, and where experience has consequences far beyond one's own desires or immediate knowledge.
Chakravarti's article on the Antiquity of Tantricism and its relation to the Vedas.
In the Shurangama Sutra the Buddha is quoted as warning that there are teachers who have been misled and continue teaching the false doctrine that the male and female organs are the means to liberation or enlightenment.
The Role of Sex
Lama Thubten Yeshe (Introduction to Tantra, 147) wrote: "There is a certain point in the mastery of the completion stage where physically embracing a consort is necessary . . . ."
There is no doubt that for most people, sexual activity provides a powerful and even sometimes, an overwhelming, experience. It is those qualities that can be used in an advanced stage of practice. Then the partner can be "used" as an activity support, or "action seal" (Skt. karma mudra.) When both partners are practitioners, then there can be further benefit.
John Powers (Introduction to Tibetan Buddhism, 1995) explains:
In these practices, one visualizes oneself and one's partner as specific deities,and one's sexual union is used as a way of generating very subtle minds.. . . . In the practices using seals, the experience of orgasm is conjoined with techniques that draw the winds into the central channel. The result is an indescribable experience of bliss and direct perception of emptiness. The partner is referred to as a 'seal' because the practice seals the realization that all phenomena are a union of bliss and emptiness. According to the second Dalai Lama, one relies on a seal "as an external condition and the experience of the compression process as the inner condition . . . ."Tantric texts stress that practice with consorts is not a form of sexual indulgence, but rather a form of controlled visualization that uses the special bliss of sexual union. It is restricted to very advanced practitioners, yogins who have gained control over the emanation of a subtle body and have awakened the mystical heat energy . . . . Those who have not advanced to this level are not qualified to practice with an actual consort; people without the necessary prerequisites who mimic tantric sexual practices thinking that they are practicing tantra are simply deluded, and may do themselves great harm.
A karma mudra (sexual partner) can also be imaginary, since one is ready for this kind of practice after having attained skill with Completion stage visualization. However, even Je Tsong Khapa who founded the Gelugpas, agreed that to attain buddhahood in one lifetime, it is necessary to use an actual consort (karma mudra) saying, "A female companion is the basis of accomplishment of liberation." That is, in order to generate an illusory body as that of a particular deity with all the qualities of existence, another person must interact with it.
Powers says further:
The only other way to accomplish this is to transform the clear light of death into a deity, but this can only be done when actual death occurs. According to Gelukpa tradition, this was the choice that Tsong Khapa made. He was concerned that some of his followers might go astray if he were to practice with an actual consort, and so he postponed the generation of an illusory body until his death. When the clear light of death manifested, he effected the transformation of subtle body and arose as a fully enlightened buddha.
Powers quotes Daniel Cozort (Highest Yoga Tantra, 1986):
"Shakyamuni Buddha, in his last lifetime prior to enlightenment, used an Action Seal, but it is said that although [Tsong Khapa] became a Buddha, he did not use an Action Seal, becoming enlightened in the intermediate state instead . . . because he feared his followers would imitate him without being properly prepared, thus hampering instead of enhancing their practice . . . ."
Also, many masters from Atisha to Kyadrubje are reported to have noted that there are few people around that can act as the Wisdom (Skt. vidya) consort, since they do not have all the required characteristics (Lessing and Wayman. Introduction to Buddhist Tantric Systems.)