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.
Let me ask you a different question.
What if we made Lara Croft into a man? It would suck. Why? Coz that character was meant to be a strong female breaking barriers. It won't make any sense whatsoever if she were a man.
That's the same as what's going on with changing the gender or sexual orientation of established characters. It doesn't make sense with the story background and motivations of the character!