Paj your post was PRECISELY what I was going to post when I saw Ashleys mistake :P
Eunuchs played a non-sexual, non-gendered role once upon a time, let's not forget.
But seriously; the argument is stupid. You can go for the moral angle and become entrapped within a what if god exists? bunch of shit, or you can go with the physical angle and the whole but what's the point? stuff.
The controversial angle; insanity. Strictly speaking, you are insane if you do not percieve the world as it is. While this is a hugely broad way of saying things, it covers things from anorexia to autism, from schizophrenia to psychopathy. We can argue that we are all made up of varying quantities of a lot of 'insanities', but when it boils down to the bare bones, being 'crazy' is to be irrational to the point of madness.
I have to admit it, that while I can tolerate transexuals, I do think that they are crazy. Crazy in the sense that I believe that they are wrong in what they think. I think I was born to be a premiership footballer -- but I'm not! So where's my operation to fix this? Oh, yeah! There's that thing called tough shit. Damn.
Essentially, chopping your penis off (I know that's not how the operation works) is a stupid idea. I think it's stupid. There, I said it. Trannies are stupid.
Bah.