In order that energy can be circulated back up to nourish the unconscious (or internal intercourse, a terminology used in the text), there must be energy to be generated in the first place. It seems to me that Taoist masters like the author Liu Huayang consider control of sexual impulse a key objective of their practice, they talked as if it is the only form of energy that will be generated! Since a Taoist (or Zen) master has to keep a celibate lifestyle, using sexual energy generated by sexual impulse does seem not to be a legitimate approach (this approach of using sexual impulse to jump-start the energy is sometimes called sexual chi-kung or bed-room technique [房中術], certainly not a technique of major Taoist meditation practice). And the master is right!
But, a very big but, there comes the misreading of the text by many people, including famous contemporary Chinese scholar/Zen master Nan Huaichin. The text was misinterpreted as: "Meditate until one (assuming male) got an erection without sexual impulse, then circulate this sexual energy up the spinal cord (du 督脈)". And many people after reading Taoist Yoga (which is more popular than Hui Ming Jing) said it doesn't work! It is interesting to note that in Nan's book Tao & Longevity (basically a book saying that Taoist Yoga doesn't work, among other negative comments to the practice in general), he reviewed that he did observe some practitioners of Zen meditation do have experience of erection (with sexual impulse) and this embarrass the practitioners and hinder their practice that needs focused attention. Nan in his book suggested a "solution" - eat less, referencing Buddhist monks eat one meal a day (presumably that prevent them from getting erection during meditation!).
To solve the riddle, we need to analyze other sources of energy together with the sexual one.