As per request. :)
This has been a post a very long time in the making. I've meant to write this up numerous times, and while this is not an issue here (as it tends to be elsewhere), it's something that I need to vent. :)
The modern Western world has a tendency to exploit things. Anything, really, especially if there's the potential for income. Whether it's the New Age cultural misappropriation of Eastern ideas, from karma to Tantra, to the mass-marketing of authentically fake items from African cultures. But, I am specifically going to aim my ire at the perversion of Tantra- literally and figuratively speaking.
Tantra isn't sex. It has very little to do sexual positions, orgasm delay or as a method to quicken kundalini. There's a whole market out there based upon 'tantric massage', or the proper way to massage your partner's yoni (vagina). You know what that's called? Diddling, or mutual masturbation if someone else is giving the helping hand. It has *nothing* to do with anything remotely tantrik. It has everything to do with fooling the naive out of hundreds of dollars. It's the same thing with these 'Sacred Sex' institutes. You pay them money so they can show you some sensually erotic form of foreplay, and sometimes watches you do it- to make sure it is being done right, of course. If you're an exhibitionist or want to add some romance to the bedchambers, it might be perfect for you. If you are wanting to learn tantra, rest assure that you are going to be screwed.
So, what is Tantra? Tantra is a form of Eastern practice, associated with a number of mostly Hindu sects, that deals with the reconcilliation of perceived opposites to a state of Oneness. Because Hinduism is panentheistic, the idea that all things exist within Deity, it is believed that no natural thing is bad/wrong/evil. And because Hinduism is henotheistic, believing that Deity is One, to announce that this is good or that is bad presents a dualism. Therefore, Tantra places an emphasis on the ritualistic breaking of taboo, of which sex for worship is but one, to delineate the boundries with the objective in raising the sadhana (practitioner) to that state of Oneness, samadhi or nirvana, and eventually liberation from the cycles of life-death-rebirth.
The most infamous of the taboos are known as the tantrik panchatattva, the five elements of parched rice, meat (to include animal sacrifice), intoxicants (alcohol or bhanga/cannabis), fish and, yes, sexual intercourse. These are culturally notorious because Brahmanic tradition, your 'mainstream Hinduism', has long had a emphasis on ritual purity. Any meat, including fish, was inappropriate for use in worship AND outside of worship (despite there being no actual Vedic edict) because violence brought about the end of the animal's life. Intoxicants are forbidden, especially alcohol, because it inebriates the senses and there's two stories where incidents occurred from the influence that both Brahma and Krishna placed a curse on it. Cooked rice was considered the damaging of the traditional offering of something that is both fruit and flower in one, yet incapable of reproducing itself- something emblematic of Oneness. Sex was wrong because worship was to Deity alone, with no division of thought to your lover.
The tantrik sadhana feels otherwise. He, or his mate (called a shakti), believes that because such things are good enough for us, then it's good enough for God/dess. Some schools, such as the kaulas, take the five elements as literal and may incorporate one or all of them in a tantrik puja. Others consider them more transcendent in nature, as personal sacrifices to achieve Oneness with Deity. I follow a kaula path as closely as I am allowed.
I mentioned something earlier about sex for worship. This is sex of tantra. Goddess is worshipped through the shakti, the female partner. Without going off on a tangent, it's assumed and traditionally a female, but there's nothing forbidding a male to assume the role, scripturally speaking. Anyway, it is Goddess that is worshipped, not the representative. This begins with an elaborate set of rituals seating Goddess into the shakti. Unrobed, veneration is given to her sex organ via mantra, mudra, and the offerings given in a traditional offering. Actual penetration may occur afterwards, however it's actually less common than assumed. Does some of that sound familiar to some of you? What if I mentioned that one traditional ritual was to meet her at a crossroads, trace a moon tattva on her forehead and invoke Goddess?
The act is not for sexual gratification. It is for uniting with Goddess to become One with Her. That goddess may be one of the ten tantrik goddesses, the Mahavidya, most of whom you would do well in not ever offending.
This is not a very common practice. Tantra suggests that you do not do anything more often than you do. Too Much and Too Little were two demons slayed by Goddess. Moderation is the key.
These are not the only Tantra elements. The ritual I posted, Bhuta Suddhi, is one of high importance. Nyasa, an action involving the placing of mantras on parts of the body to facilitate a bringing together of the sadhana with Deity, is intimately tantrik.
If it serves to unite yourself with Deity, irrespective of ritual taboo, it is Tantra. If there is a shock to your system when you do that no-no, and that shock elevates your consciousness to Deity, it is Tantra.
Otherwise, it is not.