The transgenderist enjoys being a little of both sexes, and prefers to remain in a half-way state. The transgenderist may take hormones, have some forms of plastic surgery to complete appearances, but retain their original sexual functioning even if they have the means to surgically alter their genitals. Sex is a vague polarity, as I have mentioned, but a polarity nonetheless. While intersex accidents and mutations can rarely occur, almost all non-microbial creatures on the earth are physically clearly male or clearly female. There are some exceptions, such as the earthworm, which is naturally hermaphroditic, but amongst species, clear sex differentiation is the norm.
Gender is far more of a spectrum, and even somewhat fluid.
Gender is real, it has a biological basis and is hardwired into the nervous system, but is often less dramatically polar than physical sex. The transsexual is a highly polarized being, commonly motivated by gender wiring that is at a polar extreme on the gender spectrum. The transgenderist is more fluid in gender definition than that, and can find contentment in expressing a mixed gender situation in the shape of their body. Transvestism can be so pleasurably reinforcing, however, that it becomes a permanent desire and fascination even to obsessive levels. Anything seriously pleasurable can do this.