The Roman Stoic philosopher, contempoary of Seneca and teacher of Epictetus, Gaius Musonius Rufus (1st century A.D.), was not a Christian, but his view on marriage came, as John Finnis expressed it, "particularly close to articulating" a view on marriage attuned to the natural law. Finnis, "Natural Law Theory and Limited Government," in Robert P. George, ed., Natural Law, Liberalism, and Morality (Oxford: Oxford University Press, 2002). An excerpt of his discourse on marriage was preserved by his student, a certain "Lucius," who also preserved in twenty other reasonably lengthy extracts (as far as extracts go), and which were included in Stobaeus (Floril. xxix. 78, lvi. 18).
Here is a translation of Discourse 13A by Cora E. Lutz, Musonius Rufus: The Roman Socrates. (New Haven: Yale University Press, 1942):
The husband and wife, he [Musonius] used to say, should come together for the purpose of making a life in common and of procreating children, and furthermore of regarding all things in common between them, and nothing peculiar or private to one or the other, not even their own bodies. The birth of a human being which results from such a union is to be sure something marvelous, but it is not yet enough for the relation of husband and wife, inasmuch as quite apart from marriage it could result from any other sexual union, just as in the case of animals. But in marriage there must be above all perfect companionship and mutual love of husband and wife, both in health and in sickness and under all conditions, since it was with desire for this as well as for having children that both entered upon marriage. Where, then, this love for each other is perfect and the two share it completely, each striving to outdo the other in devotion, the marriage is ideal and worthy of envy, for such a union is beautiful. But where each looks only to his own interests and neglects the other, or, what is worse, when one is so minded and lives in the same house but fixes his attention elsewhere and is not willing to pull together with his yoke-mate nor to agree, then the union is doomed to disaster and though they live together, yet their common interests fare badly; eventually they separate entirely or they remain together and suffer what is worse than loneliness.