According to the Natural Law why is it wrong for infertile people to engage in nonreproductive sex acts?

According to the Natural Law why is it wrong for infertile people to engage in nonreproductive sex acts? For example, what if a woman is too old to get pregnant, lacks necessary reproductive organs, is in an infertile period of her menstrual cycle, or is already pregnant? What would be wrong with such women engaging in unnatural sex acts with their husbands? How is this contrary to the reproductive end of sex?