The initial argument is dumb. I'm going to go ahead and translate the con argument against having NPC's available to anyone. Here goes: I'm straight and will get what I want regardless, and I'm a ittle annoyed that the character I like is available to someone else, or isn't straight like me.
Non-herosexual NPCs can and do include NPCs that are:
-straight and not available to my gay/lesbian character
-gay/lesbian and not available to my straight character
It doesn't have to be all "me me me." I
want Alistair/Cullen/Solas to be straight, even if this prevents me from romancing them, because it defines them more as people. They have specific wants/desires/preferences, same as many people in real life.
I like that Sera is only romanceable by a female inquisitor for the same reason (again, even though this prevents my Male inquisitor from romancing her)
Yes, there is "nothing a straight person can be, think, or like a gay person couldn't" and games obviously shouldn't give gay characters offensive stereotypes. But added detail, especially about something as defining as sexuality, can often help make the NPC seem more real and relatable.
Their preferences can be shown in dialogue as who they remark on while traveling, how they respond to the other companion party banter, how they respond to the player's advances/actions, help flesh out their background, etc.
e.g., Wyll. @KillerRabbit says that devils "assume the gender they think is mostly likely to get the response they are after." If this is correct, Wyll
should be more interested in women according to the rules of the universe. Making him herosexual (equally amenable to either gender PC) contradicts this story lore.
Now, if Wyll was
willing to sleep with the player regardless of gender but was less aggressive about pursuing a romance with a male PC, that'd be fine.
Edit: Wyll's patron is a cambion. But my example stands as a theoretical
