Leviticus 18:24

In all these, to wit, above-mentioned sins. Whence it is apparent that the several incests here prohibited are not only against the positive and particular law given by God to the Jews, but also against the general law and light of nature. And therefore the law about these things was one of the seven precepts of Noah. And the sober heathens condemned such incestuous marriages. The Roman historians observe, that when Claudius the emperor had married his niece, (which is one of the lowest kinds of incest here mentioned,) and the senate in complaisance with him had made it lawful for any to do so, yet there was but one, and he too an obscure person, that followed his example.