There’s two different, slightly overlapping issues at play here: representation, and opportunity.
Representation is about the idea that its important to see people like yourself reflected in popular culture. This is most often talked about in the context of race - Black Panther was a big deal in large part because its so rare to have a black person helming a giant Event Movie like that. Scarlett Johansson got a ton of criticism over taking the lead in Ghost in the Shell, which was originally written to be a Japanese woman, because again, you don’t have a lot of lead roles in major movies like that for Asians. This aspect is pretty straightforward.
Opportunity is a bit more nuanced. Because its already considered inappropriate to cast a white actor as a black character, solving the representation problem there necessarily resolves the opportunity problem: more black roles means more opportunity for black actors. But that’s not necessarily the case for other groups. Trans actors, for example, almost never get cast in cis roles. And there aren’t a whole lot of trans characters out there, so there’s very limited opportunity for trans actors already. So when a high profile role for an explicitly trans character come along, and they decide to cast Scarlett Johansson (again!) there’s a justifiable sense that she’s screwing a struggling trans actor out of a huge career break.
There’s a similar dynamic when it comes to disabled actors. If you’re in a wheelchair, it doesn’t matter how great your audition was - if the character you’re auditioning for isn’t already in a chair, they’re not going to rewrite the character to put him in one just to get you in the role. They’re just going to cast a non-disabled actor. So, when the rare character comes along that uses a wheelchair, and they cast an able bodied actor in that role…
When it comes to gays, it gets complicated, because let’s face it: gay men, at least, have never exactly had trouble getting work in Hollywood. Speaking just for myself, I have no problem with a straight actor taking a queer role. The existence of the queer role in the first place is enough of a win for me, and I’m not really interested in policing celebrities sex lives to make sure they’re sufficiently gay for a given role.
I think you’ll probably start hearing more complaints about straight actors taking gay roles in the future, just because “non-minority should not pretend to have minority status for a movie” is a much easier concept to grasp than the last four paragraphs I just wrote. But I also don’t think it will get that much traction. Because if we set a “rule” that says Chris Evans can’t ever play a gay role, that means we don’t ever get to see Chris Evans making out with another dude. And I’d be lying if I said I wasn’t interested in seeing that.