Last month, Salvatore Cordileone, the archbishop of San Francisco, made controversial changes to a handbook for Catholic high school teachers in his jurisdiction. The changes included morals clauses, one of which forbids those teachers from publicly endorsing homosexual behavior. There are plausible legal and educational objections to this move. But there is a deeper issue, one that raises fundamental questions about Catholic teachings on homosexuality and other sexual matters.

Sex can contribute to any couple’s fulfillment as human beings. Isn’t this just what it should mean to live in accord with human nature?

The archbishop has justified of his decision on the grounds that homosexual acts are “contrary to natural law.” Unlike many religions, Catholicism insists that its moral teachings are based not just on faith but also on human reason. For example, the church claims that its moral condemnation of homosexual acts can be established by rigorous philosophical argument, independent of anything in the Bible.

The primary arguments derive from what is known as the “natural-law tradition” of ethical thought, which begins with Plato and Aristotle, continues through Thomas Aquinas and other medieval and modern philosophers…

The problem is that, rightly developed, natural-law thinking seems to support rather than reject the morality of homosexual behavior. Consider this line of thought from John Corvino, a philosopher at Wayne State University: “A gay relationship, like a straight relationship, can be a significant avenue of meaning, growth, and fulfillment. It can realize a variety of genuine human goods; it can bear good fruit. . . . [For both straight and gay couples,] sex is a powerful and unique way of building, celebrating, and replenishing intimacy.” The sort of relationship Corvino describes seems clearly one that would contribute to a couple’s fulfillment as human beings — whether the sex involved is hetero- or homosexual. Isn’t this just what it should mean to live in accord with human nature?

Natural-law ethicists typically don’t see it that way. They judge homosexual acts immoral, and claim that even a relationship like the one Corvino describes would be evil because the sex involved would be of the wrong sort…

…Just trying to formulate the argument shows how strained it is. There are, of course, numerous subtle distinctions employed to defend it, requiring equal subtlety to respond. And many would see the argument as proving too much, since proponents also use it to show the immorality of birth control, masturbation and even non-reproductive sexual acts between heterosexuals.

Most important, however, the argument has no satisfactory response to two crucial questions. First, why, even if nonreproductive sex were somehow less “natural” than reproductive, couldn’t it still play a positive role in a humanly fulfilling life of love between two people of the same sex? Second, why must nonreproductive sex be only for the selfish pleasure of each partner, rather than, as Corvino put it, a way of building, celebrating, and replenishing their shared intimacy?

The natural-law argument might make some sense to those who see homosexuals as dominated by an obsessive desire for pleasure, to which they subordinate any notion of fidelity or integrity. The courageous uncloseting of many homosexuals has revealed them as people like most everyone else, searching for and sometimes achieving a fulfilling human life through rich and complex relationships. Since the official church, under Pope Francis, is more than ever open to this sensible view, the time is overdue for a revision of its philosophical misunderstanding of homosexual acts.

But should the failure of the natural-law case against homosexual behavior bother Catholics, who, after all, can also appeal to the Bible’s denunciations of homosexual behavior? Here another aspect of Catholic thought becomes crucial: The church accepts that there are two distinct sources of truth: divine revelation and reason unaided by revelation (for example, the “natural reason” of scientists and philosophers). But it also holds Thomas Aquinas’s view that there can never be a genuine conflict between these two sources. Therefore, any apparent conflict results from our failure to understand what either God or reason is saying.

Most important, there is no assumption, in any given case, that we must resolve the conflict by revising the apparent conclusion of reason. For example, the church (eventually) decided that the scientific claims of Galileo and Darwin were correct and required revisions in teachings based on biblical passages suggesting otherwise.   It is, therefore, an open question whether to accept the reasonable conclusion that homosexual acts need not be immoral and reject the view that this is what the Bible says…

… Christian scholars argue that the acceptance of slavery (even in the New Testament, by Paul) merely reflects the limited perspective of the Bible’s human authors (similar to their belief in geocentrism or six-day creation) and does not reflect God’s revelation.

The condemnation of homosexuality could plausibly be treated in the same way…

…I understand that an archbishop is not politically in a position to deny what is still an official church doctrine. But there is nothing that requires him to vigorously enforce a teaching that is so dubious even in terms of the church’s own view on the two sources of truth…

… the church needs to undertake a thorough rethinking of its teachings on sexual ethics, including premarital sex, masturbation and remarriage after divorce. In every case, the old arguments no longer work (if they ever did), and a vast number of Catholics reject the teachings. It’s time for the church to realize that its sexual ethics are philosophically untenable and theologically unnecessary.

Gary Gutting is a professor of philosophy at the University of Notre Dame, and an editor of Notre Dame Philosophical Reviews. He is the author of, most recently, “Thinking the Impossible: French Philosophy since 1960,” and writes regularly for The Stone.