Check out this article by John Corvino finding the fatal flaw in the Catholic natural law theory on the morality of sex.
Instead, the Church's main arguments against homosexuality have been rooted in "natural law," and specifically the premise that sex must be open to procreation. Thus, all deliberately non-procreative sex is sin.
Consider for a moment the implications of this premise. Contraception is an obvious no-no, given the Church's position. So is masturbation. These facts are enough to make hypocrites of many Catholics who condemn homosexuality "because the Church says it's wrong."
Also, forbidden, though far less often discussed, is orgasmic non-coital sex between married heterosexual partners, such as oral sex, masturbation of one's spouse, or anal sex. (Such acts are permitted as foreplay, but never on their own.) Official Catholic doctrine permits no exceptions here. Imagine the case of a man injured in such a way that he can no longer pursue coital sex, but still enjoys performing oral sex on his wife for the intimacy it achieves between them. It would seem permissible (perhaps even selfless and admirable) for him to engage in such sex, but the Church says no.
Thus far, at least the Church is consistent in its views. (Stubborn, perhaps--even foolish--but consistent.) But there's one implication of the "openness to procreation" premise that the Church refuses to acknowledge. If sex must be open to procreation, then it should be wrong for sterile (or postmenopausal) heterosexual married partners to have sex. Imagine a woman whose ovaries and uterus have been removed for medical reasons. Clearly, her sexual acts will never be "open to the transmission of life" in any morally meaningful way. But the Church declines to condemn such acts.
Why the apparent inconsistency? Catholic natural law theorists answer that such acts can still be of "the reproductive kind." But it is difficult to make sense of this claim, except as a lame attempt to deny unpalatable conclusions that clearly follow from the Church's position. If a sexual act cannot result in procreation and the couple knows it, then how is the act "of the reproductive kind"?
Instead of starting with the presumption, as the Church does, that all sex is bad and must be justified by being procreative, we should instead presume all sexual acts between consenting adults are morally neutral or good and then come forth with practical reasons why the acts are either wrong or otherwise not acceptable. This isn't to say, "anything goes." To the contrary, many sexual acts between consenting adults are morally wrong or otherwise should be socially stigmatized. For instance, if the couple is not on birth control and are not "fit" to bring a child into the world, such a sex act ought not be socially accepted. Just like unprotected homosexual sex between untested parties.
In many ways, the Catholic position on sex is downright immoral and irresponsible. Even married couples can only handle so many children at once. Couples ought not bring more children into a family than they can responsibly handle. There are some couples, those on the right end of the fertility Bell Curve, who will literally have over a dozen children in a lifetime marriage. And for most couples, excluding the Bill Gates and Donald Trumps who could not only support all the children but hire as many nannies as needed to assist the wife, that's just a bad, irresponsible life-plan to say the least.