I'll be honest, don't really care much for the whole 'Transsexual is an adjective, not a noun' statement. Then again, any type of linguistic prescriptivism typically rubs me the wrong way.
I guess I can see part of the reasoning, as I'm guessing anyone who says a statement like that is likening transsexualism as a transient identity or a disease. And when someone has a disease you don't refer to them as 'an autistic' or 'a disable'. Some people hear 'This person is a transsexual' and think it's functionally equivalent as 'This person is a cripple'. And then there's all these accusations of insensitivity and unwitting 'transphobism'.
Many people use 'transsexual' as a noun in much the same way as people use country names as nouns to refer to people. And although the term 'American' has its fair share of positive and negative associations, I've never heard of someone trying to change people's attitudes about Americans by telling them they shouldn't use 'American' as a noun.
If anything, I see the excessive pedantism when it comes to language as counterproductive, because it's irritating to people who don't intuitively grasp what they're doing wrong. The subtle nuances of difference between 'I am a transsexual' vs 'I am transsexual' or 'I am a transsexual person' are lost on almost everyone, and it'd be a lot more productive to change people's attitudes over changing the way they happen to speak.
