Q: Why does that character have to be gay/bi/black/Asian/Hispanic/etc?
A: As opposed to what?
I’ve found this to be a useful response, because many people will hesitate before saying “white” or “straight.” That hesitation comes from the realization, however subconscious, that they have defaulted all characters to white and straight, and are thereby declaring this normal, while everything else is other. From here, if they choose to acknowledge their internalized (unintentional but still harmful) supremacy rather than going on the defensive, they will begin to understand the real value of representation.
Q: This story isn’t about romance! Why does it matter if the characters are gay?
A: What should they be instead?
Essentially the same response. By that logic, any character in any story who does not have a romantic or sexual story arc should be aromantic and/or asexual. But the truth is, sexuality is only one part of a character’s identity (hey! just like IRL!). Any character of any race, gender, or sexual orientation can go on an adventure that does not involve sex or romance.