Nicene and Post-Nicene Fathers, Vol. III:
Moral Treatises of St. Augustin: Section 5

5. Also the question is wont to be asked, when a male and female, neither the one the husband, nor the other the wife, of any other, come together, not for the begetting of children, but, by reason of incontinence, for the mere sexual intercourse, there being between them this faith, that neither he do it with any other woman, nor she with any other man, whether it is to be called marriage. 1944 And perhaps this may, not without reason, be called marriage, 1945 if it shall be the resolution 1946 of both parties until the death of one, and if the begetting of children, although they came not together for that cause, yet they shun not, so as either to be unwilling to have children born to them, or even by some evil work to use means that they be not born. But, if either both, or one, of these be wanting, I find not how we can call it marriage. For, if a man should take unto him any one for a time, until he find another worthy either of his honors or of his means, to marry as his compeer; in his soul itself he is an adulterer, and that not with her whom he is desirous of finding, but with her, with whom he so lies, as not to have with her the partnership of a husband. Whence she also herself, knowing and willing this, certainly acts unchastely in having intercourse with him, with whom she has not the compact of a wife. However, if she keep to him faith of bed, and after he shall have married, have no thought of marriage herself, and prepare to contain herself altogether from any such work, perhaps I should not dare lightly to call her an adulteress; but who shall say that she sins not, when he is aware that she has intercourse with a man, not being his wife? But further, if from that intercourse, so far as pertains to herself, she has no wish but for sons, and suffers unwilling whatever she suffers beyond the cause of begetting; there are many matrons to whom she is to be preferred; who, although they are not adulteresses, yet force their husbands, for the most part also wishing to exercise continence, to pay the due of the flesh, not through desire of children, but through glow of lust making an intemperate use of their very right; in whose marriages, however, this very thing, that they are married, is a good. For for this purpose are they married, that the lust being brought under a lawful bond, should not float at large without form and loose; having of itself weakness of flesh that cannot be curbed, but of marriage fellowship of faith that cannot be dissolved; of itself encroachment of immoderate intercourse, of marriage a way of chastely begetting. For, although it be shameful to wish to use a husband for purposes of lust, yet it is honorable to be unwilling to have intercourse save with an husband, and not to give birth to children save from a husband. There are also men incontinent to that degree, that they spare not their wives even when pregnant. Therefore whatever that is immodest, shameless, base, married persons do one with another, is the sin of the persons, not the fault of marriage.

Footnotes

Nuptiœ

Connubium

Placuerit