Quote: After this, he begins a section on natural law, proclaiming procreation as the final cause of sex, therefore sex outside of heterosexual marriage is wrong.
Why is this never extrapolated? By that same reasoning, eating for any reason other than fulfilling minimum nutritional needs is wrong. Drinking more than is necessary for hydration is wrong. Clothing yourself for any reason other than protection from elements is wrong. Living in a house which does anything except provide basic shelter is wrong.
For all of Christianity's aversion to the fact of evolution, they sure seem determined to make sure we don't stray too far from our animal heritage.