We might imagine that the root of our marriage customs is to be found in the Bible. So let us begin our story there, with the letters of Paul, written around the 1st century A.D.
In the letters of Paul, we face a quite complicated situation. In the letter to the Romans, Paul was probably writing to Gentile Christians. They would, therefore, be married according to the custom of Roman law, and not Jewish law. This does not effect Paul's stress on the indissoluble nature of marriage, but it may well alter our perspective of quite what he means by sexual relationships before marriage.
There were various Roman forms of marriage. For the patrician class, there was the oldest Roman ceremony of confarreatio. The lower class plebeians had no similar form of marriage, and they used either a process of fictitious sale called coemptor whereby the wife was "sold" to the husband, or as a matter of implication when the wife had remained with the husband for a year. However, none of this applied to slaves - the union of two slaves (called contubernium) was simply regarded in law as nothing but a promiscuous relationship, probably because the slaves had no rights to decide how to arrange their lives; they were simply their master's property. It should also be noted that none of these "forms of marriage" constituted a marriage, which was simply a matter of consent, they were merely gave evidence of that consent.
If Paul was writing to a Roman community, and was aware of the forms of Roman marriage, then his comments on sexual relationships before marriage must be reinterpreted in the light that it was accepted marriage custom for a couple to live together before becoming married; indeed, for the plebeians, this was one forms by which marriage was evidenced!
I am not suggesting that Paul can, in any way, be so interpreted as to condone all modern sexual relationships. I am simply pointing out that his words on marriage must take into account the marriage customs of his age, and it is illegitimate to read Into Paul's letters our modern formalised marriage.
It is also worth noting that, prior to the decrees of the Council of Trent in 1563, European law held that a simple matter of agreement to marry, followed by a period of cohabitation, was sufficient to constitute marriage; no formal or ecclesiastical ceremony was necessary.
In England, it was only after Lord Harwicke's Act was passed in 1753 that a formal ceremony was made essential to marriage. This was superseded by the Marriage Act of 1823, which tidied up irregularities in the earlier Act.
So it can be seen that the modern form of marriage, as a laid down in statute binding on the individual, is a relatively recent innovation. That does not mean that it is without merits: there were problems with the system of informal marriage that needed to be resolved. But it does mean -that when we think of marriage, we are associating with it a recent social construct (i.e. modern marriage laws) which simply did not exist for centuries.
To take the words of Jesus or Paul on marriage and to simply apply them as if they were talking about our modern position on marriage is, quite simply, to misread what they were saying. They were men of their time, speaking to the problems of their time. The words of Jesus, for instance, would have been inapplicable to Abraham or Jacob, even though their marriage customs do not show the slightest hint that they were subject to divine disapproval in the Old Testament narratives. Equally, Paul was speaking to free citizens and not slaves; his doctrine of marriage simply side-steps the issue, but his remarks on slavery suggest that he would have acquiesced in the contemporary Roman laws on the matter.
When the modern defenders of marriage appropriate the Bible to support their rigorist position as representing "the will of God", they are mistaken on two counts: (I) they ignore or devalue the diversity of belief on marriage customs within the Bible; (II) they assume that Jesus and Paul were addressing themselves to modern marriage customs. I hope that I have shown how this sort of thinking is brought to the Bible, and not read out of it. It is not simply a case, as the self-appointed guardians of morality suggest, of eros defiled. It is equally, If not more, a case of dogma enthroned.
History paints a different picture and shows us how marriage customs have changed over the centuries, with by far the greatest and swiftest change being the formalisation of marriage law - a relatively recent development. We can build on that, as long as we remember that we build on shifting sands.