I think it's more nuanced than that. Context matters. Having a gay actor playing a straight roll or a straight actor playing a gay role is different from skin colour because you can't see gayness or straightness. That said, in this case I'd say "meh, I don't care" but there are cases where skin colour matters. For example, the actor who plays Nelson Mandela or Martin Luther King has to be black because their colour is central to their stories.
Agree but I can conceive of times when a play based around the life of Mandela showing him as an everyman figure could have a number of different actors playing him of different colours and sex playing Mandela, or a play with black actors playing the white roles, and vice versa.
One reason for a more 'authentic casting' position on colour is opportunity. If in the the case of Wolf Hall it was all authentic casting of colour then there would be almost certainly no roles that could be played by someone of colour. This doesn't apply as you note to sexuality. And while colour might once and still mean thar you have less chances of being cast, being gay has had much less effect.