I've seen this debate pop up so much, and honestly, I think the 'problem' label is part of the issue. Framing fanworks about same-sex pairings as a 'problem' inherently casts a shadow over them. What I notice in fandom spaces is that slash fic often becomes a catalyst for exploring character dimensions the source material ignores or suppresses. Take a stoic, canonically straight male hero—in fanfic, his emotional vulnerability, capacity for tenderness, or conflicts about duty versus desire get fleshed out through a M/M lens. This doesn't erase diversity; it adds layers.
That said, there's a valid criticism about certain archetypes repeating. The 'top/bottom' dynamic can sometimes flatten characters into roles rather than letting them be messy, complicated people. But that's less about the pairing being gay and more about the writers falling into shorthand. The best fics I've read defy those molds entirely, giving us anxious tops, confident bottoms, or relationships that fluidly move beyond that binary. So the effect isn't monolithic—it pushes diversity in some circles and limits it in others, mirroring the broader creative process.
My shelf is full of fics where a side character, barely a footnote in the original book or show, becomes a richly drawn protagonist because someone wanted to explore his potential romance with another man. That's net positive diversity, even if the road there has some potholes.