Intriguing clues about a practice from the Middle Ages, for which one could be burned at the stake:
In his monograph on The Millennium of Hieronymus Bosch Wilhelm Franger has brought together much interesting material on the Adamites. They practiced, we learn, a modum specialem coeundi, a special form of intercourse, which was identical with Noyes’s Male Continence or the coitus reservatus permitted by Roman Catholic casuists. This kind of sexual intercourse, they declared, was known to Adam before the Fall and was one of the constituents of Paradise. It was a sacramental act of charity and, at the same time, of mystical cognition, and, as such, was called by the Brethren acclivitas– the upward path.
According to Aegidius Cantor, the leader of the Flemish Adamites in the first years of the fifteenth century, “the natural sexual act can take place in such a manner that it is equal in value to a prayer in the sight of God.” A Spanish follower of the Adamite heresy declared, at his trial that “after I had first had intercourse with her [the prophetess, Francisca Hernandez] for some twenty days, I could say that I had learned more wisdom in Valladolid than if I had studied for twenty years in Paris. For not Paris, but only Paradise could teach such wisdom.”
Like Noyes and his followers, the Adamites practiced a form of sexual communism, and practiced it not, as their enemies declared, out of a low taste for orgiastic promiscuity, but because Complex Marriage was a method by which every member of the group could love all the rest with an impartial and almost impersonal charity; could see and nuptially know in each beloved partner the embodiment of the original, unfallen Adam — a godlike son or daughter of God. Aldous Huxley - essay on karezza's heritage
More from a biographer of Bosch, Virginia Pitts Rembert (Hieronymus Bosch, p.43):
She is writing about the work of art historian Wilhelm Franger, who argued that Bosch was a member of a "Brethren of the Free Spirt" sect, and that this is borne out by his "Garden of Earthly Delights:.
A peculiarity of some of the writings and doctrines of the Free Spirit movement is in their echoes of Gnostic ideas in texts such as the Gospel of Thomas and the Gospel of Philip. Imagery of the need to be 'naked' or purified before encountering salvation, the possibility of the Resurrection being a spiritual state of fullness experienced in this life rather than the next, the importance of Mary Magdalene, and descriptions of becoming drunk on the Holy Spirit all can be found in Thomas and other Gnostic writings as well as in the practises of various Free Spirit gatherings (some Beghard congregations are said to have conducted Masses nude), the Sister Catherine Treatise and the work of Marguerite Porete. A typical parallel, for instance, can be seen in the following extract from the non-canonical Gospel of Thomas, which seems to endorse the pantheism of the Free Spirit's followers:
It is I who am the light which is above them all. It is I who am the all. From me did the all come forth, and unto me did the all extend. Split the piece of wood, and I am there. Lift up the stone, and you will find me there.
Similarly, the idea that one who has Knowledge (Gnosis) or Union with God cannot and does not sin is found in the gnostic Gospel of Philip:
The one who has Knowledge is a free person. But the free person does not sin, for the one who sins is a slave of sin.
— Gospel of Philip 77:15–18
Whether these echoes are circumstantial or not is up for debate, especially as these Gospels were presumed lost until their discovery among the Nag Hammadi manuscripts in the 20th Century. It is possible that such Gnostic ideas or texts were in circulation in the areas where the Free Spirit flourished via the Cathars, whose dualistic and transcendentalist approach to Christianity had its roots in the East but this is unproven, although it is true that the regions where the Cathars were strongest in northern Europe – Flanders, the Rhineland, Cologne – were the regions where the Free Spirit spread most strongly. Whatever the case the parallels are there and very striking.