What is his reasoning, based in constitutional law doctrine, for holding this belief?
Well, he doesn't actually, er, provide that analysis.
Despite beginning his first sentence citing Hugo Black's description of the "layman's" (mis)understanding of constitutional law, Knepper demonstrates no understanding of constitutional law himself. Instead, like so many anti-gays are currently doing, he basically bleats over and over again in various different ways how it's not the job of the judiciary "to bring liberation to oppressed minorities."
Not once does he discuss due process, standard of review, or rational basis- all terms one would expect to find in a Prop 8 constitutional law analysis- in any serious way. He does include the words "equal protection clause" in his post, but he provides no analysis either way as to how Prop 8 did or did not violate the Equal Protection Clause.
The whole thing is basically, Dude, I'm gay, and I think the Prop 8 opinion was wrong.
Ironically, while playing up his gay identity as though it somehow bolsters his unsupported opinion that Perry was wrongly decided, Knepper gives us hogwash like this:
"The judge — who is gay and should have recused himself — justified his ruling in part by noting that no 'demonstrated harm as a result' of same-sex marriage could be shown, which could have made blocking it justified." [emphasis added]
Unlike Knepper, one supposes, those other gays are biased and incapable of objectivity when it comes to analyzing (or not) the constitutional rights of LGBT Americans.
Here, Knepper takes it as a given that the homosexuality of a judge is incompatible with rendering an impartial opinion on an LGBT rights matter, as though heterosexual judges exist perched on platforms of total objectivity and hold no biases whatsoever, least of all about one of the most controversial issues of our time. This is precisely the fauxbjective double standard that would demand an African-American judge to recuse herself from a case about affirmative-action or Latino to recuse himself from an immigration case, assuming that only white people could be objective about those issues. Or, that would prohibit a woman from deciding a woman's rape case because only a man could truly be impartial about it.
Ah yes, I can see it now: Lady Justices Ginsburg, Sotomayor, and Kagan recuse themselves, for obvious reasons, from federal abortion case.
One wonders, what sort of case would demand a white heterosexual male judge to recuse himself if folks with these identities are automatically impartial about LGBT rights, affirmative action, immigration, rape, and abortion? During the history of our nation's highest court, 95% of the 111 Supreme Court Justices have been white men. Perhaps Knepper thinks our forefathers were onto something by only selecting white dudes for this important judicial job until very recently.
In any event, to calm Knepper's deep concern about judicial impartiality, those who were actually intimately involved with the case, including the opposition, saw Judge Walker's sexual orientation as a non-issue. It's too bad a gay man can't.