Here’s a quote from a good history book on marriage:
“From the sovereign character of the family Roman law drew the conclusion that it was not the state that made a marriage a marriage, and not a religious organization, but exclusively the marrying parties themselves, their mutual love, their will, their agreement. Nuptiae solo affectu fiunt, nuptiae consensu contrahentium fiunt, consensus facit nuptias – such was the basic position of Roman and Byzantine, ecclesiastical and civil law in the first 8 centuries of Christian history. Moreover, in more ancient times the religious form of marriage, confarreatio, was necessary not to make marriage valid, but for manus, that is, for the acquisition by the husband of authority over the wife.
“But if marriage is concluded by the marrying parties themselves, then in what does the task of the State in relation to marriage consist? Only in verifying its existence for itself, only in registering the marriage, to the extent that this was necessary for the resolution of various questions of family and inheritance law. And Roman law left it to the will of the marrying parties to choose any form of marriage they liked, contenting itself with the minimum for its own verification.
“In ancient Rome there existed a view with regard to marriage that was the opposite of our own. We have a presumption that those living together are not married. In our time a married couple must itself prove with documents, witnesses, etc., that it is in lawful wedlock. In Rome, by contrast, the presumption was that those living together were married.
“Every permanent sexual relationship of a fully entitled man and woman was seen as a marriage. ‘We must see living together with a free woman as marriage, and not concubinage,’ writes the noted Roman jurist Modestinus. Therefore it was not the marrying parties that had to prove that they were in wedlock, but a third interested party had to prove that there existed some kind of impediment which did not allow one to see this living together as marriage. To put it more briefly, onus probandi lay not on the spouses, but on the third parties. Only when there was a basis for thinking that it was in the family or property interests of the parties to present a temporary relationship as marriage was the question of the formal criteria of marriage raised. But even in this case Roman law contented itself with the minimum. For this it was sufficient, for example, to show that there had been de facto living together for a year, the testimonies of witnesses that the parties had indeed agreed to marry or to call each other Mr. and Mrs., that some kind of marital rite had been performed, the presentation of documents with regard to the dowry, etc. In a word, speaking in legal terms, in Rome the participation of the State in the conclusion of a marriage did not have a constitutive, but only a declarative character.
“Byzantine legislation adopted the same point of view until the end of the 9th century. The constitution of the Emperors Theodosius and Valentinian in 428 says that for the validity of marriage neither a wedding feast is necessary, nor documents on a dowry, nor any festivity, since no law hindered the marriage of fully entitled people. Marriage acquired validity by means of agreement and the testimony of witnesses. Although Justinian, in his novella 74 of December, 537, prescribed that middle-class people should go to church to conclude their marriage, this demand was based on considerations, not of a religious, but only of an economic character, which is indicated by the fact that the very separation of this class of people was in accordance with their property census. And indeed, Justinian demanded that middle-class people should go to church not in order to be crowned, but only in order to draw up a document on marriage in front of an ecclesiastical lawyer and three or four clergy as witnesses. But even this formality did not last long, and on December 11, 542, novella 117 (ch. 4) freed even middle-class people from this obligation. Only upper-class people (illustres et senatores), again for reasons having to do with property, had to write documents on the dowry, while the lower classes were not obliged to write any documents at all. In the same novella 74 (chapter 5), Justinian gave the significance of an optional form of marriage, not to crowning, but to the oath ‘to take as my wife’ while touching the Bible. Only in a legislative collection of the 8th century, more precisely: in the collection of 741 of the iconoclast emperors Leo the Isaurian and Constantine Copronymus known as the Eclogue, was a blessing as a juridical form of concluding a marriage mentioned for the first time.