How did what we might have once thought was simply the obvious conclusion of reason and common sense, namely, that marriage is the union of a man and a woman, get so quickly and decisively overturned in the minds of so many?

Reports that a slim majority of Americans still oppose homosexual marriage fail to explain why so many people do not, apparently, oppose it.

The real question is: How did anyone ever come to imagine that unions between persons of the same sex could possibly be thought to be marriages in the first place?

The answer to this question, I think, lies in the surely now-incontrovertible fact that marriage in our society is no longer thought of as necessarily having anything to do with children. And this idea that marriage does not necessarily involve children — or at least the possibility of children — in turn itself goes back to a decision that our society has gradually but now almost universally been making as a result of the advent of modern methods of contraception.

Yes, contraception.

The Christian tradition had always unanimously condemned artificial methods of birth control; this was true up until the year 1929, when the Church of England broke ranks at its Lambeth Conference that year and allowed the practice in “hard cases.” Within about 20 years nearly all of the Christian communions in the tradition stemming from the Reformation followed the Anglicans, as Western society itself overwhelmingly embraced birth control as a great new modern benefit.

The Catholic Church, of course, declined to join in the general rush to accept and employ contraception. Thirty-five years ago, Pope Paul VI, in his encyclical Humanae Vitae (On the Regulation of Birth), even warned that disastrous consequences would follow from the moral acceptance of the artificial separation of the life-giving from the love-giving aspects of marital intercourse.

Now that a whole generation has passed in which contraceptive use has become as common and unquestioned a component of modern living as, say, television or air conditioning, we are now in a position to assess the results. They have been disastrous; Paul VI turned out to be right.

Even among Catholics — whose acceptance and use of contraception, following the lead of dissenting theologians who persuaded most people that the Pope's teaching did not have to be followed — contraceptive use today appears to be parallel to that of practically everybody else. And one of the principal consequences of the moral acceptance of contraception, of course, is nothing else but the sexual revolution itself.

The sexual revolution would not have been possible — it would have been inconceivable — in the absence of the availability and the supposed efficacy of contraception in preventing any “unwanted” consequences stemming from the new untrammeled sexual “freedom” and “expression.”

If we can licitly separate the life-giving potential of sexual union from its love-giving character — and modern society has long since decided that we can — then it strictly follows that the pleasure-giving aspects of sexual union become divorced from their intrinsic meaning and purpose as far as nature is concerned.

Henceforth, anything goes — as in our society today, everything having to do with sex does go!

Most people are inclined to laugh or even to sneer at the idea that this state of affairs could ever have been brought about chiefly as a result of the acceptance by society of contraception; but the fact is that it is impossible to see today's widespread moral acceptance of sodomy and so-called homosexual marriage, for example, as stemming from any other source. For ideas do have consequences. There is an underlying logic in all this that cannot be gainsaid.

Cardinal John Henry Newman once observed that people tend to think and act on what their true premises are, even when they are scarcely conscious of what they are; and our premises today include the fixed and firm conviction that there is “nothing wrong” with contraception.

But this is to affirm that there is nothing wrong with the use of sex without regard to its possible life-giving powers, as in the case, precisely, of sodomy. Most people do not spontaneously rise up and condemn homosexual marriage today because, deep down, they have accepted the idea that there is “nothing wrong” with uses of sex divorced from nature's intended purpose for sex, which would include the possibility of children.

The idea that there is any such natural use of sex, though, has now been largely abandoned in our society: It's whatever we decide. Furthermore, marriage, too, has become whatever we decide, and, in the process, marriage has lost its true identity.

Although stable marriage and the family are actually the basis of any stable society, to accept as so many do today that unions of homosexuals could somehow in any way constitute a marriage is to evacuate the very idea of marriage of its real meaning.

It is no exaggeration to say that that is where we are today: So-called homosexual marriage became possible and perhaps even inevitable when society decided that the life-giving powers of marital intercourse could morally and legitimately be separated from the love-giving powers, in other words, when society rejected Pope Paul VI's encyclical Humanae Vitae.

Kenneth D. Whitehead is the author, among other books, of One, Holy, Catholic and Apostolic: The Early Church Was the Catholic

Church (Ignatius Press, 2000).