Heterosexual acts are ordered to the natural functions of the body, in totality. <<< Not for individuals who are sterile, whether permanently or temporarily, whether due to disease or senescence, or morally indistinguishable physical barriers, whether chemical, spatial or temporal (NFP). But even stipulating to your impoverished physicalistic, biologistic, deductivistic, legalistic approach, all you will have argued is that one situation is optimal. It does not follow that other situations are gravely evil, only that they are suboptimal. Well that's enough stipulating to an anthropologically bankrupt moral philosophy for now.