47 Again, Jacob the son of Isaac is charged with having committed a great crime because he had four wives. But here there is no ground for a criminal accusation: for a plurality of wives was no crime when it was the custom; and it is a crime now, because it is no longer the custom. There are sins against nature, and sins against custom, and sins against the laws. In which, then, of these senses did Jacob sin in having a plurality of wives? As regards nature, he used the women not for sensual gratification, but for the procreation of children.
For custom, this was the common practice at that time in those countries. And for the laws, no prohibition existed. The only reason of its being a crime now to do this, is because custom and the laws forbid it. Whoever despises these restraints, even though he uses his wives only to get children, still commits sin, and does an injury to human society itself, for the sake of which it is that the procreation of children is required. In the present altered state of customs and laws, men can have no pleasure in a plurality of wives, except from an excess of lust; and so the mistake arises of supposing that no one could ever have had many wives but from sensuality and the vehemence of sinful desires.
Unable to form an idea of men whose force of mind is beyond their conception, they compare themselves with themselves, as the apostle says, and so make mistakes. Conscious that, in their intercourse though with one wife only, they are often influenced by mere animal passion instead of an intelligent motive, they think it an obvious inference that, if the limits of moderation are not observed where there is only one wife, the infirmity must be aggravated where there are more than one.
Source: Reply to Faustus the Manichaean (New Advent)