Traditional sexual morality can be stated in a single sentence, that covers all the wrinkles: any sex whatever outside of marriage is utterly proscribed. Thus the only sort of sex that is traditionally permissible must transpire between man and wife.
Weddings are elaborate affairs, sure; but that is only because traditional marriage is so definite, and so simple. It is easy to know when one is married, and easy to know when one has reneged on the marriage vows, or has otherwise sinned sexually. But it takes a Big Ceremony to nail down the permanent personal and communal commitment to such a sweeping definition.
A wedding is a passage from one order of being to another altogether larger, more difficult, more expansive, glorious, and dangerous than mere individuality. Fitting, then, that it should be so charged, huge, expensive, and total.
In a way, liberal sexual ethics are easily stated, too: under liberal sexual ethics, any sex outside the bounds of mutual consent is utterly proscribed. But here, the simplicity of the statement is achieved by skating past the great complexity of one of its terms: consent.
The difference between the two, that gives rise to the terrific complexity of living according to liberal sexual ethics, is in the form of the consent to agreement to sex that all parties thereto rely upon. The conservative version is traditional marriage, which (among other things) constitutes permanent consent to sex. Under the constraint of the sexual consent entailed in traditional marriage, consent is irrevocable, and constant; and it may therefore be assumed, taken for granted, expected, and obliged. Sex then is the “marriage debt,” that each partner forever owes to the marriage, and so to the spouse. It is a given.
The liberal version of consent per contra is always temporary, evanescent, and must therefore be renewed at each encounter (even within liberal “marriages”). It may never be assumed, or taken for granted, or expected, or obliged. It is not easily ascertained, or verified. It is subject to ex post revocation. It is never a given, but rather is given again with every iteration of the sexual act. Such giving is not thereby rendered ignoble, or less precious. But it is rendered insecure, difficult, chancy – which is to say, risky, costly, worrisome, vexed.
So we see that liberal sexual consent must somehow or other therefore evolve toward explicit and irrevocable ex ante legal agreement, formalized in a written publishable contract that is registered with the public authorities. Only thus might the consent be safely recognized, or therefore effectual in covering the risk of the parties thereto. It must, i.e., tend toward some replication of the public formalized rites of marriage. These are costly to undertake, and it would be tiresome to rehearse them at each sexual encounter. That would tend to frustrate sex (& procreation), by increasing its cost. But avoiding them would increase the legal risk of sex, and thus its cost. That too would tend to frustrate sex (& procreation). So it is that under the prevalent liberal sexual morality of the modern West, reproduction – and marriage, and even sex – have collapsed.
Excursus: Any construction of valid marriage as somehow or other dissoluble undermines its permanence and irrevocability, and so starts society on the slippery slope toward the easy criterion of liberal consent, and so to a failure of society to reproduce itself. If marriage is dissoluble at all, it is dissoluble per se; and is not, therefore, marriage in the first place, but rather only the evanescent unreliable stupid consent of liberal sexual ethics dressed up hypocritically in the skin of tradition, to lend it some specious gravitas, some convenient simulacrum of concrete inarguable reality. Legal or ecclesial recognition of divorce is then the first step toward social and biological extinction.
So traditional indissoluble marriage is inevitable. Societies that use it will outbreed those that do not.