Recently, one of my students asked me a question, “Is it true that there is semen retention in Vajrayana (in the practice of karma–mudrā of the Completion stage), but is it not so in Indian tantras?”

First of all, I think that it is wrong to sacrifice a human nature to religious corporations, dividing it into Tibetan and Indian one. There is a tendency to think that if you are a Tibetan, you can retain semen, and if you are an Indian, the practice must necessarily be different. The  retention  of ejaculation in the practice of maithuna or karma–mudrā is an allegory. In Indian kaulācāra, this kind of practices implies ejaculation, as an analogue of pūrṇāhuti in agnihotra. After which, this substance is mixed with wine and then used in pūjā. There is a lot of evidences of that fact, for example, in the Guhyasamāja-tantra:

विण्मूत्रशुक्ररक्तादीन् देवतानां निवेदयेत्।
एवं तुष्यन्ति सम्बुद्धाः बोधिसत्त्वा महाशयाः॥२१॥

viṇmūtraśukraraktādīn devatānāṃ nivedayet।
evaṃ tuṣyanti sambuddhāḥ bodhisattvā mahāśayāḥ ॥21॥

It is necessary to offer the secretion: urine, semen, female bodily fluids and offer them to the Deities. Thus, it will satisfy the great awakened ones, buddhas and bodhisattvas.

Prior to that, there is a recommendation to practice intercourse with a beautiful young woman. In order, for example, to offer semen in pūjā, it is obvious that there must be present a finalised ritual of maithuna. And there is quite a bit of such recommendations in Vajrayana. These kinds of transgressions in Vajrayana, which seems to be full of  savagery to the common man, in fact, are not much different from saptamakaras in aghora, which took a lot from traditions like kāpālikas.

If you read a description of vajrolī or amarolī in the Haṭha-yoga-pradīpikā, you will see that there is a recommendation to mix the released semen with ash and apply it as a tilaka. That is, it means that ash is a symbol of amṛta, as well as semen. It says of the same principles as in the description from the Guhyasamāja-tantra and other Buddhist texts. Here is a very similar fragment from the Yoni-tantra:

भक्त्या द्रव्यं जपेन्मन्त्रं जप्त्वा मैथुनमाचरेत् शु्क्तोत्सरणकाले च शृ्णु पार्वति सुन्दरि
योनितत्त्वं समादाय तिलकं क्रियते यदि शतजन्मावर्ज्जितं पापं तत्क्षणादेव नश्यति ॥20-21॥

bhaktyā dravyaṃ japenmantraṃ japtvā maithunamācaret 
śuktotsaraṇakāle ca śṛṇu pārvati sundari
yonitattvaṃ samādāya tilakaṃ kriyate yadi
śatajanmāvarjjitaṃ pāpaṃ tatkṣaṇādeva naśyati ॥20-21॥

The sādhaka should chant the mantra and enjoy the ‘substances’ during the practice of maithuna. Listen, beautiful Parvatī, if a sādhaka makes himself a tilaka with ‘yoni fluids’ after an orgasm, then he will instantly forsake all the sins associated with a hundred births. (20-21)

The requirement given here is very similar to the one from the Haṭha-yoga-pradīpikā. Probably, the goal here is not to retain semen so that there is no ejaculation at all. If the process is contemplative, then such a suspension or extension of the act occurs naturally. And the practitioner accomplishes that not by themselves and not because their ego wants it, like they can feed it by being ‘good technicians’. Here, the Goddess also controls the process through you. And the prolongation of the act is needed more in order to satisfy the Goddess in a woman, so she will bestow siddhi through her satisfaction. And at the end of the ritual, pūrṇāhuti is being done in the form of an orgasm. A prolongation of the act may occur due to the desire to satisfy the Divine. But not at all in order to demonstrate some kind of  ‘athletic abilities’ to stretch time, in which many people mistakenly believe. Each practice has a main task that must not be forgotten, otherwise it loses its true purpose and value, no matter how exclusive the technique may look. As we can see, ūrdhvaretā could be understood as  retracting substances back, but that is not by drawing them back into the genitals. That could be, for instance, the communion in the form of tilaka (as ūrdhva is the head area), and so on. And the most important thing is what happens on a psychic level, psychic vibes are significant. For a yogin who understands the essence of this practice, the physical side may be less relevant. They can perceive the sexual energy of life and flowering in their essence. Therefore, for some, a yogic practice is simply the essence of such ceremonies, even without their external implementation.