Yes, according to most modern secular laws. But one answer I have heard is that the Biblical law against incest was not given until Leviticus 18, so those who lived beforehand were a sort of special case.
Second, I have heard that Natural Law, which is written in the conscience, was always in effect, but must be such that it must have allowed for marriage within three degrees of separation in the case of the early generations of men. (Degree here is counted as yourself, your prospective wife, and all persons up to and including your nearest common ancestor; in the case of Mrs Cain who also happened to be Cain’s sister Adam’s Daughter, the third degree would be counted 1. Cain, 2. Adam, 3. Adam’s daughter, so 3 degrees.)
Third, I have heard is that the DNA of early generations was flawless, so the whole inbred birth defect thing wasn’t an issue then.
