It's a Pornographic term, taboo and indeed a slur if used outside of pornography to refer to a character, but using it to describe a transgender person would indeed be a degrading hardcore slur.
"Avoiding the use of slurs in general, when talking about Queer/Non-Normative Media such as this is an adjustment for a lot of people. "
Who dictates what's a slur and what's harmless porn terminology and tags? To me futa has been and always will be just a term to refer to pornography of a certain kind of intersex person. To respond to your last point, how right is it to expect everyone to adjust their terminology for a certain group of people and not have such done in kind?
Look, just say "It's not so much offensive as it is annoying and inaccurate" and I can buy your argument, otherwise it's just seems that's yet ANOTHER popular word that everyday people use that we have to "readjust" ourselves with just because a few bad apples do it.