In regards to Dorian's character development, it has to do with his sexuality. Whereas making a character player-sexual or bi for that matter, the writer can focus on other things that matter to that character besides their sexuality. You get to focus more on them as person rather than what gender they are attracted to.
What gender they are attracted to can
be a part of their person: backstory, personality, and relation to other in-world characters. Why should writers not place [at least some] focus on sexuality? Sexuality is a big part of real life after all, and so stories that ignore it are by definition going to have less potential than stories that include it. As shown by the Dorian example: if he was playersexual, then the story couldn't (not without being immersion breaking) use a plot point that depended on him being strictly gay, and in doing so use that real-world allegory. Would you prefer that Dorian's story was changed in order to make him playersexual compatible?
I agree that a character's sexuality shouldn't be their entire personality, but it's not an either/or. Writers can include sexuality and "other things that matter to that character besides their sexuality" as aspects to a character.
