This to me is like saying instead of casting Marlon Brando in The Godfather, they should have picked an actor with real Mafia ties. Or perhaps more to the point, they shouldn't have cast Randy Quaid in Brokeback Mountain because in real life, he's not homophobic.
Frankly, I'd rather have an actor as good as Heath playing Ennis than a poorer real-life gay actor in the same role.
To be logically consistent, shouldn't the same people saying that Jack and Ennis should have been played by real-life gay actors also say that the original story shouldn't have been written by Annie Proulx? After all, she's not gay, and even if she was, she would be a lesbian. What could a lesbian possibly know about male gays?
Personally, I'm still upset that the role of the mountain in Brokeback Mountain was played by a non-Wyoming mountain in Alberta.