Try this experiment -- your book opens with: "The well-dressed lawyer entered the waiting room, frowning, sheaf of legal documents in hand."
Okay, now how many of you pictured a lesbian, wheelchair-bound dark-skinned woman after that sentence? Or did you picture a generic tallish white guy, almost certainly straight? That's what Delany means, when he talks about the 'mythical norm'.
It takes skill to draw a picture of protagonist in such a way that it flows with everything else the first few paragraphs are doing. But if you don't do it, and your protagonist isn't a member of that mythical norm, then your reader will have a certain amount of mental whiplash when they realize they've mis-assumed.
It's not the end of the world if they experience that, but unless you're trying to make a political point at the reader's expense, I'd generally recommend avoiding mental interruptions that will throw the reader out of the story. Anything that might keep them from turning pages is deadly!