Via PrideSource: Dustin Lance Black (pictured; image via ABC) wants trans people in trans roles:
First and foremost, when I'm casting any role, I'm gonna look for somebody who can bring a part of their experience to the role. They still have to be a great actor, so if I can't find anyone in the world who shares some experience that they're about to portray in this character, who's also a good actor, then I'll happily go for someone else. And the big surprise is, it was not hard to find amazing trans actors and actresses to play these parts. What was difficult was deciding who to cast because so many great tapes came in. So, I call bullshit on Hollywood if they say it's difficult. And if they think it's difficult, then they should call our casting directors because they found unbelievable trans actors and actresses, and it was actually tough to decide who to cast.
Regarding why an out gay actor didn't play Harvey Milk in Milk in 2008, he said:
Years ago, it was difficult to find openly gay actors to play openly gay roles — that was difficult. When we were doing Milk, that's what we said we wanted to do, and the studio gave us full permission to do that. So, we called agents and manager friends and they all said they didn't have any gay actors or actresses, which is funny since I knew some of their clients were gay!