An objection, as to how an unlawful intermingling with the daughters of Cain could be charged against the line of Seth before the prohibition of the law.
Germanus: If that command had been given to them, then the sin of breaking it might fairly have been brought against them for their audacity in so marrying. But since the observance of that separation had not yet been established by any rule, how could that intermingling of races be counted wrong in them, as it had not been forbidden by any command? For a law does not ordinarily forbid crimes that are past, but those that are future.
Source: Conferences (New Advent)