Lord Sumption emphasised that the doctrine does not apply merely because the subject matter discloses that a state acted unlawfully, and there are further public policy exceptions under fundamental human rights and fundamental norms from which no derogation is permitted. (ukscblog.com)
That is the Public Policy exception, and, given some locations, cults, groups or states that would accept marriages between closer blood relatives than others, or ages, etc.. (nystateofpolitics.com)
The Public Policy exception permitted the Home State not to recognize a marriage from a Foreign (another) State, where the type of marriage seeking recognition was «repugnant to the public policy of the forum state.» (nystateofpolitics.com)