A fundamental requirement of a judge is impartiality. He is to be as impartial as an umpire or a referee. His responsibility is to take rules written by others (including and above all the Constitution) and faithfully and neutrally apply them without bias or favoritism, and without changing the rules in the middle of the game to give the advantage to the team he happens to like best.
Tim Donaghy, an experienced NBA referee, was recently banned for life when it was revealed that he placed bets on games he himself officiated. He eventually plead guilty to federal conspiracy charges and is in prison as we speak. You just can't have a referee - or a judge - who has a built-in bias towards one team or the other.
A homosexual judge cannot help but give the home-field advantage to every legal team appearing before him who represents homosexual causes. It will be impossible for the visiting team, the team representing sexual normalcy and natural marriage, to get a fair shake in his courtroom.
Apparently a heterosexual who sits on the court doesn't have a unique viewpoint or a possibility of having his or her judgments clouded by his or her unique life experiences. It isn't worth worrying that he or she will have an insufficient appreciation of the struggles that sexual minorities (or minorities in general) face, yet homosexuals are obviously impeded in coming to an objective verdict.
Of course, the only people who are competent- whose judgments will not be impeded by their own unique history- are normal people. That's why we call them normal: they're the default. Putting anyone aside from a heterosexual, white, upper class male, who alone among the classes have the unique ability to distance themselves from their upbringing and enter into a realm of pure reason, is simply irresponsible.
It's the same song and dance we heard during the Sotomayor hearings. My letter to the editor of the New York Times on that issue summed it up as well as I can:
It is disingenuous to paint Judge Sonia Sotomayor’s commitment to bring a unique point of view to the court as evidence of racial bias, as leading Republican members of Congress have done.
Proponents of a diverse bench believe that pursuit of the objective truth requires the input of many different kinds of people. We cannot allow the nation’s highest court to become blinded by a myopia brought about by racial, class and gender homogeneity. The upper-class white male is not a tabula rasa from which verdicts come untainted by circumstance of birth.
The court will be all the better for a panoply of viewpoints and backgrounds on the bench.