Do not have sexual relations with any woman until you comprehend, in meditation and internal experiences, that this is the person your divinity wants you to be with. This is why dating exists: so you can get to know the other person before you commit a serious mistake.
Also, you should be a gentleman and not approach your partner with lustful dominance. Conscious love develops through patience, charity, compassion, selflessness, and chastity.
If you look at the scriptural citation you provided, the original Chinese is ambiguous. Adultery is not accepted in our tradition. The problem is that many Taoist texts have ambivalent meaning, or have been outright corrupted. We do not recommend anyone go around "mounting young girls," because to lustful Westerners, this interpretation is a source of great suffering.
Sexual magic can only be practiced within legitimately constituted homes, says Samael Aun Weor. A legitimate home is established by your Innermost. If you do not know the will of your Being, then be patient until you do. Otherwise, acting on one's own desires will lead to problems.
For thirty years I sought God. But when I looked carefully I found that in reality God was the seeker and I the sought. -Bayazid al-Bastami