[Oliver_W]

I don't really think there is a "right" amount of any demographics that should be in a movie, especially if we're talking on screen. But when it comes to "big" professional productions there is a wrong number - zero.

Production companies shouldn't feel the need to go out of their way to be diverse, but if they're hiring on a meritocracy and who's available , the diversity will come naturally. But if a production ends up being staffed entirely by straight whites, that's almost certainly deliberate.
(indie productions obviously shouldn't be held to the same standards)

Bit of a tangent, but it's relevant - bigger productions "need" to have big names in their cast, and LGBT people might not be readily available , especially if they don't want to out themselves.