Recently I came across an article called “The man does not part his robes, nor does the woman loosen her belt”: Controversy over Non-Contact “Sexual” Alchemy, by Clark Hudson, an Assistant Professor of Religious Studies in University of Virginia. The Professor’s contention is basically that non-contact “Sexual” Alchemy is a (failed?) attempt of compromising two dominant practices: Solo inner alchemy and for-real Sexual alchemy, with the latter form being more powerful!
An academic approach on religious matters suffers because the practitioners don't look at things that way. A Buddhist thinks a Christian at best being irrelevant, and verse versa. If an academic thinks that a Buddhist and Christian are talking about the same “God” in different veils, both the Buddhist and the Christian will consider him rather irrelevant!
Practitioners of Taoist yoga never denied the health benefits of sexual chi-kung. As per Taoist Yoga (classic text by Zhao Bichen 趙壁塵 translated by Charles Luk), chi generated through sexual chi-kung can better the body but will be of no use for further spiritual practice (because further practice requires a mind free from physical attachments). Actually sexual chi-kung practitioners had no issue with this saying. The clients (or students) of these sexual chi-king practitioners were usually (as one can imagine!) rich males who wanted to go beyond the limitation of male sexual constrains, and it would like god-send when a “Taoist master” told him that he could do it and at the same time improve his health! Any connection with Taoism could certainly gain respectability and credibility of such practices. Moreover, these clients (or students) were most likely not interested in further spiritual advancement!
Our professor did rather thorough research, and he mentioned one concept: muck alchemy 泥水丹法 in passing. According to the Wu-Liu tradition (五柳派), sexual chi-kung is a form of muck alchemy- it means this kind of practice will be of no use to further spiritual advancement. It can however also means after one session of sexual chi-kung, a male practitioner will become a piece of muck (!) Well, fair to say, unless he also practices semen retention (the detail of which can be found in Master Mantak Chia’s seminal books) at the same time. Perhaps our professor would also like to have a hand on such practice too, but of course that will go beyond what is required for being an academician. And I rest my case.