Dermot O’Callaghan made the claim during a debate on plans by the British government to introduce gay marriage at a meeting on the fringes of the Conservative Party conference in Birmingham.
Mr O’Callaghan said heterosexual marriages last longer if both partners stay sexually faithful.
However, he said, it “is extraordinary” that research shows “that the very opposite happens” if a gay couple attempts to do the same: such a relationship is “more likely” to be shortened and less stable.