After looking into the matter, I'm afraid that the website we run on does not include the option for anything other than female, male, or other/unlisted genders.
I certainly understand where you're coming from with respects to those who do not entirely identify with male or female genders, adding a genderfluid option isn't in our cards right now, without entirely updating the forum. Just as well, we aren't entirely sure if there is a mod for the SFM that offers this as it is, since part of the purpose of having a neither male/female option is partly covered in this respect.
So, this is a very helpful and kind gesture, but I'm afraid this one isn't in our cards at the time. Thank you for the input, though!