What's wrong with making someone gay or female for the sake of being that? That's a way to create depth for a character, and even if it doesn't add much to the story it helps bring a sense of liveliness to the world to give mundane details. Also, it seems that in your mind someone being straight or white or male or whatever is the default, so if someone decides to do forego that they're going to automatically get extra scrutiny and will either pass or fail the seemingly arbitrary test of whether or not this diversity is acceptable.
Because it involves co-opting an established IP and changing something to suit the ideology of people who don't like or respect the IP to begin with. It's betraying the people who are the fans of the IP for a shallow politicized reason. This usually results in a poorly written story and characters which makes people dislike this stuff more.
If someone wants characters with certain traits, they can create their own original IP with a story that makes sense with those characters. Just leave other people's shit alone and everything is hunky dory.