I think there are a few reasons why it seems so black and white.
1. They kind of approach an understanding of something (like convergence in math) where you build up a series of this is true, this is not true little by little until you see the whole picture. Unfortunately because it's very theoretical and is largely based on sociology which is a huge huge huge thing to try to understand, the chances of ever seeing the whole picture are slim.
2. There's a language barrier. There are only so many ways to describe something without leading to confusion. You're talking about trying to describe feelings that one person has over feelings that another person has. By the time you get done explaining it all, nobody's going to remember what the original purpose was. So they come up with labels to group similar ideas together to make them easier to work with. If you start adding too many labels, nobody's going to be able to remember them all and psychology will start having lots of splits like the sciences do, where people can only specialize in very tiny sub-sections and never fully understand the whole subject. And I don't mean psychology as the whole subject. The whole subject may end up being transsexualism with 10 different sub-sections of specialization and the same goes for the other labels that fall under transgender (and any other area of psychology for that matter).