I am not sure what the hypocrisy is here, nor do I see the connection between opposing gay marriage and cheating on your wife.
If gay marriage is 'damaging to the social fabric by devaluing marriage' (to paraphrase the trite conservative argument), then adultery must be
doubly so: where gay marriage can effect only a
theoretical marriage, adultery,
by its very definition, occurs within the boundaries and limits of an already-established marriage - one cannot be adulterous if one is a bachelor. Hence the hypocrisy:
Herr Duvall, who has in the past crusaded to keep marriage from being devalued,
actually devalued a marriage -- his own. The specter of gay marriage is a theoretical bugaboo; fornication is a very real and really damaging thing.
Duvall, just now, did more damage to the institution of marriage than all of the gay marriages the world over.