Just to provide an easy reference, Ed Feser has written about this:
Nor does any of this entail that we cannot use F when we know its end won’t in fact be achieved; for in that case we are not using F for the sake of frustrating the realization of E, and we are not ourselvesattempting to frustrate the realization of E in the course of using F. To foresee that F’s end E won’t in fact be realized is not the same thing as using F in a way that will prevent E from being realized, any more than foreseeing that something will happen is the same as causing it to happen. Hence there is nothing inherently wrong with sex during pregnancy, or during infertile periods, or with a sterile spouse, or after menopause. And there is nothing inherently wrong with using broad mental reservations -- which do not actually convey falsehoods but merely express truth in an oblique way -- even though one knows that one’s listener will in fact probably draw the wrong conclusion.Nor does anything said here entail that man-made devices are per se contrary to nature in the relevant sense. The reason contraception is objectionable from a natural law point of view is not because it involves the use of drugs in the case of the birth control pill, or artifacts in the case of condoms. The use of drugs to treat impotence, or of eyeglasses to improve vision, are not “unnatural” in the relevant sense, despite the means being artificial, because they do not involve using a faculty contrary to its natural end. Indeed, these means restore or enhance the faculties’ power to realize their natural ends. By contrast, the “withdrawal method,” though it does not involve the use of any artificial devices or drugs, is unnatural in the relevant sense, because it does involve using a faculty contrary to its end.Nor is it any objection to “perverted faculty” arguments to point out that some people have very strong desires to use their faculties in a way contrary to what natural law theorists claim to be their natural ends, even if these desires have a biological basis. That something has a biological basis doesn’t by itself make it “natural” in the relevant sense, since some biologically grounded traits are defects. For instance, color blindness and Down syndrome have a genetic basis, but that hardly makes them “natural” in the relevant, A-T metaphysical sense. By the same token, even if it turned out that homosexual attraction or a compulsion to lie had a genetic basis, that wouldn’t show that these desires are “natural” in the relevant sense.