The author doesn't mention one time that the result of unencumbered sexual activity between adults is procreation.

Religions generally reserve procreation to occur within a structured family unit.

In my personal belief system, the family structure has a fundamental place in our eternal progression and thus the sanctity of family is of the utmost importance. The 'rules' regarding sexual activity are entirely related to the family structure.

I would bet that most religions that have prohibitions on sexual activity do it for very similar reasons.