Do you know who decides that? You do, at your gaming table. 5e said in the PHB that anyone can be any gender or orientation they want. There's your rule and guideline. The NPCs aren't specifically called out as heterosexual or cis. Their orientation and identity is left blank on purpose, because as the PHB tells you, they can be whatever you want them to be.
Sure, nowhere in the D&D rulebooks does it say that a certain percentage of your NPC characters must be homosexual, transgender, of "African" culture, or of "Asian" culture, or be of anything really. And? But the rulebooks do (now) make it clear that the official game is an inclusive and welcoming one. If your table doesn't buy into that, well, as long as you are all happy no one's really going to notice or care.
However, increasingly, NPCs in published game products ARE having their gender orientations being explicitly called out, alongside their fantasy race, cultural background, and other personality characteristics. WotC is doing it, Paizo is doing it, and I'm sure other RPG companies are doing it as well. Because they don't want to be insensitive or exclusive to anyone who might want to play D&D and find themselves mirrored in the fantasy world. They also want to do what's right, for the sake of doing what's right, increased sales be damned. I've noticed that Seattle game designers tend to be a fairly progressive bunch, on the average.
If your table doesn't swing that way, well, okay. My table? We're confronting changing societal views on those who are "different", both in our games, in our workplaces, and in our communities. And we're slowly crawling towards being better people for it.
I'm white, male, and hetero, as are most of my gaming friends. Most of our PCs have also been white, male and hetero. As have most of our NPCs. Acknowledging non-binary gender in our games is a relatively new thing for us, and we still don't do a lot of it because our norms are, well, white, male, and hetero. Most of my non-white, non-male, and non-hetero gaming friends have been quite used to our old-fashioned norms and don't let it bother them over much. But as we see increased focus on giving legal rights, tolerance, and respect to those who are "different" in the mainstream media, we also see it in our games, and we've embraced it. We feel that we are better people for it, and that we are viewing the world more clearly and without the tainted lenses of discrimination, fear, and hate (or "uncomfortableness" if "hate" is too strong a word). We've got a long way to go, as it is still to easy to fall back on assumptions we grew up with, but we're making progress.
Don't care to include that kind of growth in your games, or in your life? Okay. I hope you continue to play and enjoy D&D and participate in the hobby. But I'm glad the "gatekeepers" of our hobby, the large (relatively) RPG publishers, don't see things your way.
Gotta call BS on this one. If a game table says they don't want to discuss sex in their games at all, that doesn't target GLBT any more than anyone else. Most of our games don't talk about sexual orientation or identity because it's not relevant to why we play the game. That doesn't mean we're telling GLBT players they are not welcome. No sexual aspect of the game is discussed.
Sigh. I'll repeat. SEX =/= GENDER.
My table's games don't get all that sexy. We generally don't roleplay sexual encounters and romance plots, at least not very overtly. But we acknowledge that GENDER has always been a part of our games, in the sense that most of our characters are heterosexual (and male to boot). ALL of our NPCs have had GENDER, as in the heterosexual "norm". We don't fool ourselves that GENDER has never been a part of our games.
We're not "social justice warriors" or crusaders at my table (
gods, I *hate* the misuse of that term as a pejorative, alongside the canard of "politically correct", but I digress) and we're not designing our PCs or our games at large to fill some affirmative action quota mandated by WotC. We are acknowledging that in reality, many folks don't identify as heterosexual and/or cis-gendered, many of those folks are our friends, our family members, and our community members. We are acknowledging that if our fantasy games are going to mirror our modern world, especially in an aspirational sense, that to ignore the truth of nonbinary gender is a negative thing.
Again, if your table doesn't swing that way, that's fine. Well, as long as you don't have anyone playing with you who is homosexual, transgendered, or has a homosexual or transgendered family member, friend, or co-worker and isn't quietly put off by ignoring that facet of reality.