The trans* community are not the only ones to point out that "transsexual" is not a noun, it's an adjective. There are many other groups who have made the same point. It's rude to refer to someone as "a dyslexic", "a spastic", "a gay", "a black", "a Chinese", "a Malay", "a ginger".
Why is that? Well, for one thing, the characteristic being referred to is not the only significant thing about them; it's not a defining characteristic. They may also write poetry, drive a vehicle, tap-dance, sing, be a great lover, and so on.
There are plenty of alternatives to using these adjectives as nouns. A transsexual person (who could be male or female - the term transsexual does not signify gender); a gay person; a person with ginger hair / a ginger-haired person; a person with dyslexia / a dyslexic person; a person with cerebral palsy; a Black person; a person of colour; a Chinese person; a Malaysian person. Of course, their ethnicity is only relevant in the context of a conversation about ethnicity. (Though several Black people have pointed out recently that the whole colour-blindness trope is actually really unhelpful and assimilationist.)