I'd say this is crazy but then I remembered that hentai netorare/cheating is one of the mo...

and the fact NTR/cheating is popular in BL fiction is so sad

This isn’t even cheating (unless the brother didn’t know the husband liked MC and was fucking MC).

But it is cheating bro. This whole plot literally about cheating brother. MC knows he's cheating and still commits to it, from third person perspective aka ours its cheating. Its not cheating only for this married couple cause they see cheating as fetish and enjoy it.

Cheating brother? Did the brother cheat on his husband? HUHH .

Oh are you trying to say about MC, knowing that his brother’s husband is fucking him, but hiding it from his twin?

I think it’s not really a fetish(?), but more like the twin brother and his husband both like the brother. It wouldn’t be cheating for them, so not a cheating fetish. You mean cuckold fetish, that’s the closest thing.