I'm just curious, why would you find it offensive to belong to a "3rd gender" category? Nothing of how you think or act or dress would need to change, it is simply defining what you are for better understanding (which is pretty much the entire job of words, to communicate).
Obviously the "3rd gender" would have a large variety of types under it with the current way I was trying to define it... but, then again, the same is true for male and female.
A 3rd gender is already used in other parts of the world (india and thailand i believe, though they have their own issues... it is at least a socially integrated category).
I could go into the reasons that needing to define trans people is important (such as dating), but... you already know all that.
Basically, the entire reason would be to not mislead others. If you are on a dating site and someone looks at your profile saying you are a woman, chances are they might have issue with that when you start getting to know eachother (having extra bits in bed is a big tipoff for the pre-op people). Unless you think it makes more sense to always have to "clear things up" as you get to know someone. Always needing to add in the fine-print at some point with people.
The only issue with a 3rd gender category is the entire dynamic of "stealth" goes away. Which (in our current world) is kind of dangerous sometimes (so I completely understand the practical NEED to blend in with male or female). But, in places like the forum you are at (where they already know what you are), the need to do that goes away and you can simply be what you are (which, again, is lacking fitting words in our language).
mmm, just thinking out loud

Maybe the he/she is enough? *Shrug* logically it just doesn't seem to want to fit in my mind.