There's a clear element of fantasy inherent in the futa fetish, wherein the old guard of people that wanked to it knew it as a fantasy. Indeed, calling one a 'futa' and not a 'girl' would seem to imply a clear difference.
Now, for your average coomer, there's still fantasy in trying to pretend two chicks with dicks going at it is yuri - namely, that their dicks don't make them any less women, and that there are actually trannies out there that look like that. Futanari is notoriously just drawing an otherwise normal (well, for exaggerated hentai) female body and then slapping a prick on right above the cooter. That obviously is not how a BEAUTIFUL TWANS BODY looks.
It's a very subtle rebranding, but a clear and obvious one - these idiots had trouble separating themselves from the porn they beat it to, and they're on a crusade to make it all "real". That's why there's such a furor over something that the rest of us would clearly respond to with "huh? who fucking cares?"