As an LGBT person, I can see why this is disappointing. Truth is there are so few big gay roles in stuff that yes, from our community's point of view, it would be great to see a gay person getting that role. Doing so is no threat to straight actors, who have 90% of roles as straight characters to choose from.
Yes, I know that gay actors play straight roles too - but it wasn't too long ago that actors (Male and female) wouldn't dream of coming out as it would actively damage their careers and believability in straight roles. It's great they can now, but the shadow of it still hangs over it all. For example, Jim Parsons wasn't out for years, well into playing Sheldon.
I think it's hard for straight people, who see themselves reflected on tv and film every day, being played by people with the same sexuality as them to understand fully why gay people would be disappointed by it. But it's like Scarlett Johansson playing both Asian and trans roles as a white cis woman - I wouldn't automatically see the big deal, but that's because I'm white and cis so it doesn't effect the visibility of my community. But I have to recognise that's my unconscious privilege making me blind to the struggles of other communities as it doesn't directly affect me.
Representation really does matter.