Diversity has been lacking in our fiction for too long. This has been said many times, by many people, but it bears repeating. This history is what has created the white/straight/cisgendered/able defaults.
After all, where does a "default" come from? It's an expectation established by a long history of stories told without diversity - a long history where the stories told by other voices were marginalized or silenced. It's not just a conscious expectation, either - like many of the other patterns in our lives, it becomes so ingrained that we stop noticing when we see it.
Our psychological expectations are very powerful. Once we've learned a pattern, we learn to correct it. Take listening to language as an example. If we are on the telephone and have a bad connection, we can still understand a lot of the conversation because our knowledge of language will fill in the blanks. Or if we're speaking with someone and that person sneezes or makes a speech error or adds some other extraneous noise, we know how to filter it out and return to the fundamentals of the message.
When it comes to diversity in fiction, this psychological tendency is a real problem. People who are writing diverse worlds often have to fight their own unthinking smoothing of diverse patterns - smoothing which is born of their understanding of the pattern of the milquetoast past. Similarly, people who read in diverse worlds may not notice their diversity. The expectation of the default can cause readers to see the default in what they read, even when it is not there. In other words, we've been handed all-white, all-straight, all-cisgendered, all-able fiction for so long that when we're handed something different, some people will go so far as to subconsciously erase the descriptions that do exist in the text. Thus it can happen that when some readers see the actual diversity of the text represented visually, they cry out in surprise (and often enough, disappointment).
Right now, we're in an exciting time for fiction. Stories are starting to be diversified, and a great many people are working actively to achieve this. A lot of these people are members of the historically marginalized groups in question, but quite a number are members of traditionally more privileged groups. (Privilege is a complex concept because it exists simultaneously on so many different parameters.)
I've heard people say that it shouldn't be necessary to specify things like skin color, ethnicity, sexuality, etc. in some contexts. I disagree, particularly given the strength of the expectation we are fighting against. If we are not explicit in the ways we portray diversity, they are far more likely to be ignored or elided by readers.
Yes, it's possible that people who try to diversify their fiction will do it badly. That's why books like Writing the Other by Nisi Shawl and Cynthia Ward are so important. Fortunately, the more people try, the more people are likely to start getting it right, and changing that expectation by creating a new pattern. And it's about time we started teaching people to see diversity in fiction, because it will help the cause of equity and empathy in our own societies.
Here are a couple of valuable questions to ask yourself if you are designing a fictional world:
1. How does diversity happen in a society?
What types of diversity are genetic and exist already within a given region? What types of diversity occur between and within cultural subgroups of this region? What other regions exist in this world for people of different backgrounds to come from, such as travelers or traders, or royalty who marry for alliances, or people of different nationalities come together for a major project (public works or space travel, etc.)?
2. How is diversity envisioned within a society?
Once you've answered the question of how to get readers to envision the diversity of your world, it's worth taking the extra step and asking how the different populations of your world envision the diversity around them. Some cultural groups (often insiders or the privileged) can be blind to the existence of others. Some can be more explicitly aware.
Take a look at your work and the diversity you are trying to portray with an eye for how integrated that diversity is into the world and the story - how *essential* it is to an understanding of the world works, so that readers will be less likely to miss it. Think of yourself not only as portraying diversity, but as teaching readers (and characters) how to perceive the diversity of this world.
It's something to think about.