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.
One of the things Corvino misses is that sex from the Catholic perspective has three purposes and any of these are sufficient to make sex OK if they are pursued in a way that doesn't involve deliberately interfering with the other two. The three purposes are (roughly) procreation, due remedy (what is sometimes called by the very uninspiring and somewhat misleading name of 'marital debt'), and union. The case of a sterile woman clearly can fall under due remedy (which has to do with one's partner's needs) and union (which has to do with preserving the marital bond); and ex hypothesi the fact that any particular sexual act is unable to lead to procreation is both out of the couple's hands and an incidental feature of the persons involved. If the sexual act is performed for either due remedy or marital union, in a way consistent with both, and with an attitude toward the sexual act that would welcome children if they were possible, there is no reason on the Catholic view that it should be condemned.
This is all very basic. Nonetheless, I find the argument interesting in that Corvino incidentally brushes up a genuine flaw in common attempts to build a sexual ethics out of natural law. The moral ends of sex are noticeably versions of another set of moral ends, namely, the moral ends of marriage. And it seems clear that any attempt to discuss sexual ethics can't move anywhere unless it faces squarely and clearly the moral status and nature of the marital bond. It's very clear, I think, that Catholics often think they can skip this step; and it's just not possible to do so without distortion. In particular, it's silly to assume you can pull a natural law argument against homosexual acts out of the purpose of procreation like a rabbit out of a magician's hat, without examining how procreation relates to the conjugal bond. That's not the way it works; natural law is by definition much more systematic and rational than that, and doesn't allow shortcuts. The case of sterility, for instance, shows that the issue cannot be inability to procreate as such, as many people seem to assume.
