In order to do a better job defending marriage we have to do a better job defining it. What is marriage anyway? Because false understandings of marriage are common, even among conservative Christians, we are frequently caught flatfooted when it comes to things like the gay marriage debate.
A marriage requires two components or elements. The first is old school sexual intercourse and the second is a socially recognized set of vows, committing the couple to a legally recognized and protected state of marriage. If one or the other is missing, then so is the marriage.
To use the language of philosophy, each of these is a necessary condition for marriage (without which, not), but not a sufficient condition. In other words, you can’t have a marriage without the presence of both of these elements, but the mere presence of one of them does not constitute or create the marriage. The absence of either will result in no-marriage, but the presence of either does not automatically result in marriage. You must have both together.
The first element is the one flesh union (Gen. 2:24). And the first thing to note is that it is possible to be sexually immoral without a one flesh union occuring. This occurs, for example, with porn, or with heavy petting, or with oral sex, etc. So the one flesh union is not defined by the presence of one or more orgasms; it is defined by a heterosexual sexual union, as classically understood. If Christians allow their definitions of the one flesh union to broaden, such that it includes “messing around,” the problem is that homosexuals can mess around also, and in exactly the same ways. Only heterosexuals can be married because only heterosexuals can perform the central act that is necessary to the establishment of marriage. Homosexuals can’t be married for the same reason that bolts are useless without nuts — key equipment is missing.
The second element would be the vows, legally recognized as such. By vows I do not mean the promises that Billy breathes into Sally’s albaster ear in the back seat of the car, to wit, promises to love her “forever and a day.” I mean vows and promises that are recorded, however their society records such things, and which are enforceable should one or the other of the parties try to walk away from their promises.
Marriage is a public, social act, and not just a private sexual one. More completely, it is a public act that depends for its authority upon a private sexual act, one that will be performed shortly after the reception.
If a couple have sex without the vows, they are not married. They are not “married in God’s sight.” They are one flesh, sure, but that by itself is not marriage. Paul says that a man who sleeps with a hooker is one flesh with her (1 Cor. 6:16), true enough, but that does not make them married. If a young man seduces a girl, they are one flesh, but it is still open question whether they will get married or not (Ex. 22:16-17; Dt. 22:29). Particularly in the Exodus passage, you can see that the reality of the one flesh union does not force the father’s hand — and if the young man in question is a 14-carat schlub, it must not force the father’s hand. The father does not “have” to give permission on the basis of wrong assumptions about them already being married in some mystical way.
So then, in summary, if a man and woman have sex, it does not make them married. If another couple exchange vows, but never have sex, then they are not married either. In the latter case, if a couple separate because of his impotence (say), that is what annullment is for. That would not be a divorce, properly speaking.
You can see how these definitions help us when we work through how various relationships might end. A man visiting prostitutes should repent and stop it. A man sleeping with his girlfriend should repent and stop it. If marriage to her is wise and/or lawful, he should then marry her. If it is unwise or not lawful, then they should break up, and not look back. A man who exchanged vows with a woman who then (for whatever reason, it happens) would not let him sleep with her should seek an annulment. A man who exchanged vows with a woman and had sex with her is . . . married. And in order for that relationship to end lawfully, it would have to be a biblical divorce — for infidelity or desertion.
So there you have it — old school marriage.