Sex instinct or libido is the primary energy source of man and animals alike. We can't imagine it to be otherwise, since all existing living things are winners in the genetic wars of the species with a successful procreation record backed by strong enough libido drive. Man, being different from animals, has evolved a long way through cultural development. The primary driving force may still be libido, but its manifestations or various sublimation has developed beyond our easy comprehension.
One interesting question: what is the mechanism of sublimation of man's sexual energy? The best place to look at is to study the behavior of primitive men just beginning their first baby steps.
Primitive men were able to have excess libido and were able to channel such excess energy into cultural activities, like organized hunting and organized cultivation of the land. Rites and rituals were abound. For primitive men just starting to have a budding consciousness, it was easy to communicate to their unconscious and channel their excess libido using symbols and direct it to better, cultural usages.
The following passage by Carl Jung succinctly showed how it worked in one case (from his seminal essay: On Psychic Energy)
"The transformation of instinctual energy is achieved by its canalization into an analogue of the object of instinct. Just as a power-station imitates a waterfall and thereby gains possession of its energy, so the psychic mechanism imitates the instinct and is thereby enabled to apply its energy for special purposes. A good example of this is the spring ceremony performed by the Wachandi of Australia. They dig a hole in the ground, oval in shape and set about with bushes so that it looks like a woman’s genitals. Then they dance round this hole, holding their spears in front of them in imitation of an erect penis. As they dance round, they thrust their spears into the hole, shouting: “Pulli nira, pulli nira, wataka!” (not a pit, not a pit, but a c_____!). During the ceremony none of the participants is allowed to look at a woman."
It is interesting to note that "during the ceremony none of the participants is allowed to look at a woman". In Taoist yoga practice, the objective was said to create an embryo from internal copulation, the male part of one combined with the female part of one, and with one's sexual energy as driving force for the whole practice. So much so, erection has been specifically seek for as THE moment to transfer such energy in reverse direction for inner copulation. And, a very important concept here, the practitioner is asked NOT to have erotic thought, so that such libido or sexual energy will be sublimed for higher/cultural purpose. In the case of the Australian primitive, sexual energy will be sublimed for future day cultivation of the soil; and likewise for Taoist yoga practitioner, sexual energy will be sublimed for future day cultivation of spiritual immortality.
One more similar point, in the case of the primitive Australians, their undeveloped consciousness allowed their unconscious to ready "buy into" the symbolism of their ritual; in the case of Taoist yoga practitioners, their meditation state (in the zone) allowed their more developed consciousness to play second fiddler, so that symbolism can work on their unconscious.
In both cases, should erotic thought be entertained at THE moment, the libido or sexual energy will be channeled into the usual way, as Taoist texts kept reminding practitioners to avoid!
I hereby rest my case that sexual chi-kung, without using symbolism and sublimation, is quite irrelevant to classic Taoist yoga practice.
Needless to say, whether it is advisable for modern man to practice Taoist yoga faithfully according to the texts (follow to the letter) is another question (incidentally Carl Jung advised against it, but of course this is HIS opinion only). Or one might prefer to phrase the question differently: to what extend should a practitioner learn or adapt these profound practices to make them more relevant to the modern man. I believe the latter formulation can bear fruit.
PS: For comparison, to direct energy to cultivate the soil, most modern men only need a small dosage of will power, instead of needing to excitedly dancing around a hole!