"So it occurred to me, or at least it occurred to the conscious part of my brain, that the only way to write non-white, non-male, non-straight characters is to read more of them."

While this may be an option, I don't think it's the best option. If I(a white guy) want to write a black character, or a gay character, or a female character, then I need to be able to crawl into their head and understand what motivates them, what pisses them off, what makes them hurt. I think the best way to get that insight is to have discussions and interactions with real people, rather than rely on the characterizations of another author.

It's the difference between eating a freshly cooked meal and eating the same meal after it's been refrigerated for three days and reheated. It loses something.