I think growing up we are sold this romanticized idea of what an artist does. In fiction, artists are always geniuses driven by pure emotion, chasing "the vision". And the vision is a kind of divine inspiration that should be followed, but not questioned. And I think that contributes to this idea of "forcing diversity".
If we treat white, straight, male characters as the default "vision", that is completely unbiased and comes from this Magical Place Where Ideas Are Born, then we see any idea questioning that as an attack on artistic freedom.
So, a male protagonist is an artistic choice, a female protagonist is "artist forced to pander to feminists" or whatever else.
Another interesting myth is the separation of visuals and storytelling. In my opinion, they're one and the same thing. Especially in comics. You do a lot of the storytelling through the visuals, the character design, the environment.
Unfortunately, better storytelling requires research, and questioning your own ideas and preconceptions, which is something I only started seriously doing in the last couple of years. And that makes the job more difficult, and a lot of people have a knee-jerk reaction against it.
Truth is everything you do as a writer/artist is a choice. I think a good example of how people only examine that on the surface is how we like to justify our choice by a different, completely arbitrary choice.
"Why are all your characters white?"
"Because my story is taking place in a magical realm inspired by Medieval England."
Whether it's even accurate or not, it's still a choice. I doubt you were instructed by a booming voice from heaven to choose that location for your story.
Ultimately, what matters to me is creativity, and diversity of the worlds we build is part of that. Also, it's easier to immerse ourselves in stories like that. If you play a video game and notice that all npc's look the same, after a short time it just gets boring, especially in role playing games, and it throws off any suspension of disbelief.
And I've never heard people talk about giving a game more character models or more available vehicles as "forcing diversity" (maybe some have, the internet is a crazy place).
Now, as far as my own comic, I think it isn't bad, though if my works were moving faster, it would be more diverse, as the story so far didn't have much space for talking about my characters private lives. But in my head I generally have already planned out what orientation they are, what is their religious position and such, and I think it helps me make better informed decisions about their behaviour.
Oh, and as far as the ability to write diverse characters, I think it's most important to have diversity among the creators, that helps to avoid tokenism.
But also, I have different kinds of characters, and I relate to each of them on some level. I think "write what you know" should mean "take time to learn about something before you use it". It also helps to have real life people that matter to us. If you don't consider any real life women strong, then you won't write "strong female characters" as complex human beings. And so on.