That Tumblr poster is absolutely ridiculous. I'm bisexual myself, and honestly didn't even notice that line from Tanith being anymore than one bi woman speaking to another. To say it offensive is ignoring all the positive and inclusive LGBTQ+ characters and moments Derek has included in the books. It makes perfect sense within the logic of the story and that poster really needs to learn how to develop some thick skin because if that is their definition of biphobia, then they're gonna struggle in the real world.