As much as people don't mean to hurt anyone's feelings most of the time, this so-called "joke" is offensive. I know people like to joke around & whatnot, but this can hurt people, even if they know you were just joking. If you use this as a joke, please be more cautious of how other people feel & only use it if the other person is fine with it, if the person is not then don't say things like this. I hope you have a nice day/night.
I am not Nonbinary nor Genderfluid, but I do think this really is a problem. I am not trying to be offensive or rude, but honestly though, it's not that hard.