It’s weird. Way back in the day (like, pre-Voltron Legendary Defender era) the relationship between two unrelated fictional guys being considered “brotherly” would have been quite validating to us slash shippers. It’s like, oh wow, they’re that close, huh. They already consider themselves each other’s family. They’d die for each other. We’d have thought a romantic relationship stemming from that was perfectly logical!
And it’s not like the ship would ever be canon, so having that sort of deep bond was the next best thing. The fanworks would just have to take it from there.
But it’s so different these days. A character says “you’re like a brother to me” and now shipping two unrelated fictional guys is literally incest? Even though this has been a trope in straight romances for forever? Wow, the view has really shifted! Of course, shippers back in the day were much looser regarding canon roles, ages, power dynamics, or hell, even whether or not two characters existed in the same universe in the first place! But this is one argument I’ve just never been a fan of. Like, I used to say my first gf was my sister all the damn time because I didn’t know how else to interpret my feeings for her early on. And the fact that this seems to disproportionately affect m/m ships and f/f ships is telling, too. Also, people’s views of each other can...change? Sure, maybe Character A once saw Character B as his brother, but something is different now, and so he has to go and navigate those feelings. I mean the whole point of fanworks is exploration. For the characters as well as the writer. And just like back in the day, the ships are rarely canonized, which means something is always being changed or expanded. We just didn't have killjoys in our ears telling us we couldn’t ship two unrelated fictional guys because one of them once referred to the other as a brother or whatever. Well, we did, cause homophobes exist, we just didn’t usually hear it from fellow shippers.