Sex Classical Tantra and Neo Tantra
Most Classical Tantra masters pre 4th century were celibate.
Sexual energy was used, however it is not the same thing as being horny or feeling sexy. It was a deeper expression of that ability to create life, the spark that was expressed in a holistic expression of self as a being created to procreate.
Kundalini was experimented with however only after rigorous training was this brought about (there is much confusion about this today, with most people believing their body awakening is actually a kundalini experience.)
Classical Tantra was not a mainstream householder experience it was really left to the more esoteric experimenters of life. They were very disciplined and yet extremists. They pushed every envelope there was physically, emotionally, energetically. They were loners for the most part as well, only taking an occasional student here or there to pass along the lineage they held.. the information from centuries before them.
Most people know about Tantra from the post Classical time when it began to morph into different schools. Right handed, left handed, Hindu Tantra, Tibetan Tantra, Buddhist Tantra, and a few more. Some of these used sexual rites in different ways as an affirmation to life and for use in transforming sexual ( horny) energy into something elevated and used for consciousness raising.
They were very specific rituals and not used unless a very solid foundation was mastered. Of course this got distorted by some as well as there were others realizing that they could capitalize of this exotic experience. Neo Tantra was created.
Neo Tantra was brought to the everyday householder as a way to assist in making the sex lives of a couple more fulfilling and frequently we confuse Tantra and the Kama Sutra, So does history.
One of the biggest misunderstandings in Tantra today can be seen by this statement made by Heinrich Zimmer, when he learned about Vamamarga a sexual ritual of Neo Tantra that may include any or all of the elements of ordinary ritual, either directly or substituted, along with other sensate rites, like a feast (representing food, or sustenance), sexual intercourse (representing sexuality and procreation), the charnel grounds (representing death and transition) and defecation, urination and vomiting (representing waste, renewal, and fecundity). There were other things as well.
He viewed this as a world affirming attitude and stated,
“In the Tantra, the manner of approach is not that of Nay but of Yea … the world attitude is affirmative … Man must approach through and by means of nature, not by rejection of nature.”
This very simply statement that was then grossly misinterpreted later has created the belief that is fostered today that in Tantra anything goes.
Even in Neo Tantra anything goes is not accurate until the 20 century.. It is only in New Age Neo Tantra (western Tantra) that anything goes.