Apparently there was as Newsweek article (haven't seen it) questioning the ability of gay actors to play straight characters. Might it be the viewer's perception? Is the viewer being pulled out of the show's world by thinking that actor's gay? There have been times that I've been watching a film starring a recently deceased actor and find myself thinking: He's dead. Kind of distracting. Imagine how unenjoyable Doubt would have been to watch if you spent the entire movie thinking She's a Baptist, he's a Jew, she's a whatever.
Ever have the experience of seeing a movie and then seeing it again after learning one of the actors is gay? Do you look at that actor's performance in a different light? Do you suddenly see "gayness" coming through? This has nothing to do with your views on homosexuality or the actor's abilities. It's your (the viewer's) ability to suspend reality and be drawn into the world of the show.