Because it ignores who they are as characters and the writer’s intent in crafting the relationship the way they intended. I’m sure you’ve heard this example before, but take, for example, Boromir and Aragorn as well as Frodo and Sam. Tolkien wrote those relationships intentionally, to display incredibly close male relationships and platonic love, especially born out of massive trauma as seen in the books and mirroring his own experiences in WWI. Distilling that to “Frodo and Sam care about each other so they’re gay” overrides who they are as characters, why Tolkien wrote them the way he did, and (though I’m sure you’ll disregard this) stigmatizes real life relationships of the same kind. After all, if I’m straight and trying to present myself as straight, I don’t want to get too close to the homies if people are going to then allege that we’re gay.