I'm reading Qigong Meditation: Small circulation by Yang Jwing-Ming. I'm finding this book to be really useful in further refining my understanding of Taoist energy work, particularly on one issue which concerns the retention of semen. If you've read any of Mantak Chia's work, Chia argues that it's absolutely essential to retain the semen during sex in order to avoid losing your energy. I've never been fully satisfied by Chia's explanation, mainly because I haven't fully understood the correlation between the loss of semen and the loss of energy. However in Yang's book this correlation is explicitly explained in a manner, which explains what the issue is, and also adds a further step to the process of refinement. Yang explains that when the semen is full the hormones stimulate the brain and generate sexual desire, which in turn energizes the person...and that the goal of the inner alchemy is to actually transform the semen into Qi (or raw energy) Having sex too frequently can deplete the amount of semen available to convert into qi, while not having it too frequently actually slows down the production of semen...so it seems having a somewhat active sex life is actually important for the inner alchemical conversion process.
When the semen is turned into qui, it can then be used to nourish the body. In fact, what is called marrow washing is essentially a process of taking converted qi and using it to heal/regenerate the body.
The explanation offered does provide some insight into how Taoist inner alchemy works, and specifically what the function of semen is for the energy work being done by the male (though you rarely find much written in regards to the female). I'll be sure to update readers further as I continue reading and/or trying out the techniques in this book.