"There is another one, the so-called “Secret Offering” or “Copulation Offering”, as follows"

"Both the buddha-father’s nature of expediency and buddha-mother’s nature of emptiness wisdom operate together without any difference (The buddha-father and buddha-mother copulate and both stay in the same awareness state of lustful pleasure and emptiness generated from the dual operations). This is the greatest and original copulation. The great lustful pleasure generated from such copulation can make all deities in the altar satisfied. Those deities can know many couples of the buddha-fathers and buddha-mothers copulate (many visualized couples copulate and stay in the state of lustful pleasure and emptiness.). In case of a single celestial lady, she must transform the celestial stick of hand-seal from the buddha-father or her clan master into the great lustful pleasure of copulation. By that way, one can complete the wisdom of great bliss, and observe that the wisdom of emptiness and bliss is generated continuously. Then, one can take the same buddha’s pride in following all deities’ behavior on the desires. [158: 225]

While succeeding in the visualization of Secret Offering, one must let the visualized buddhas, bodhisattvas, etc., who embrace female consorts, derive great pleasure from the obscene touch. One must make offering in that way to all buddhas, bodhisattvas, gods and dharma-protectors, etc. For example:

On the first page of The White Horse Head Vajra Practice of The Vajra Tantra, it states: “A ‘Sha’ seed-syllable at the secret place (lower part of one’s body) becomes the same green hayagriva vajra as mine. The buddha-mother’s secret place has a vajra pestle with blue and white colors at one end, together with a pig head at the other end. She holds a small drum and a human skull, and visualizes offering happiness.” Here, both the male and female deities’ secret places are specified obviously. In the completion stage of tantric practice, the pestle (penis) and lotus (pudendum) stand for the male and female sexual organs respectively. Both of them either operate with each other (to observe the bliss and emptiness in union during copulation) to do business (perform various sexual actions) or make offering (to visualize that the offering of lustful pleasure during copulation makes the buddha-father and buddha-mother happy). Everything comes from those two secret places (everything being created from the couple’s copulation). That offering is called the bliss of Secret Offering, which can also be concluded as offering the great bliss of the fourth-joy by drawing, thrusting, and moving the pestle and lotus (the male’s and female’s secret places). [34: 213]

Not only did Yogi C. M. Chen say so, but The Great Sun Sutra, the basic tantra of both Japanese Esoteric Buddhism and Tibetan Esotericism, also had stated so much earlier. For example, as mentioned in Vol. 3:

Secret Master! All Thus-come Ones in the worlds clearly understand the expedient paramita (perfection). They know the nature of discrimination is emptiness. Nevertheless, by the power of the expedient paramita, they act conditionally outside but stay in the unconditioned state inside. They show the dharma in the dharma-realm for sentient beings in order to let them stay in happiness and make a delightful resolve, or let them live long, enjoy the activities of the five desires (the lustful desires from the five sense-objects) and make offering to the buddha with such lustful activities (offering such activities on the five desires to the so-called “buddha” of Tibetan Secret Schools). The evidence is shown as above, but it is not believable to the ordinary people. [Taisho Tripitaka, Vol. 18]

It also states in All Buddhas Realize the Great Mahayana Samatha Teaching Sutra, Vol. 2, as follows:

It is great! My original nature is pure and all defilements are generated naturally. Because the original nature is clean and away from all defilements, the defilements can be used to subdue themselves. In Vol. 4, the visualization is: “Right after coming from the Thus-come One’s heart, the vajra holder generates numberless Thus-come Ones from one’s great vajra seal. Then, those Thus-come Ones merge into one and the pictures of the vajra seal holder playing with the great female consort (the pictures with various kinds of copulating postures) appear. The vajra seal holder, who wears various magnificent clothes and accessories like the vajrasattva (diamond-like person), has various kinds of subtle deportment and tightly hugs a female consort. The seal holder stays at the left moon disc in Buddha Akshobhya’s altar and recites the following verse: “It is great! I have the best subtle offering to all buddhas. Because I know the offering of this lustful pleasure, I can transfer this offering to all buddhas.” [Taisho Tripitaka, Vol. 18]

Even when the lustful pleasure is generated during the practice of Couple-Practice Tantra, one must visualize offering the obscene pleasure to the buddhas as well. In All Buddhas Realize the Great Mahayana Samatha Teaching Sutra, Vol. 8, it states:

Both bodies copulate tightly; the natural subtle happiness becomes the offering; by devoting this offering, one can become vajrasattva soon. The sexual contact corresponds with the real subtle love so that the lustful pleasure of touch is generated anywhere in response to the contact; by offering that to all buddhas, one can get the vajra jewel. The solid pleasure is held continuously (lasting long without leakage); every response to a touch generates the greatest pleasure; by offering that to all buddhas, one can attain the vajra dharma. The lotus (pudendum) and pestle (penis) copulate to generate the great subtle pleasure everywhere; by offering this to all buddhas, one can obtain the vajra karma." [Taisho Tripitaka, Vol. 18]

source:
[link to dhammacitta.org]