We have transsexuality. Why, besides it being almost impossible to pronounce, do we not transgenderuality? Or transgenderity?
If you follow the "logic" of those who argue that it's "transgender", not "transgendered", we need both. But we only have the one -ity. Heck, we don't even have a transgenderité! (Perhaps the French do? But they always have the stylish words and funky philosophies.) :-)
Transgenderuality isn't quite right. Okay, it's an abomination that should not be acknowledged. Either that or it's a boy band that wears frocks? Transgendity? That sounds like one of those vacuous post-modern philosophy's. "The transgenditists were, today, eradicated by a lotion..." Well, it's either or you can only hope it isn't catching. Or contagious. I hear gayness is contagious; exposure to gay folk automatically leads to you to being gay, or a gay lifestyle. ... It's quite clear that some Evangelists need happier company.) [Sorry? :P ] And transgendists are... Well, not transsexual separatists. A transgendist would be a "person who supports transgenderism"? Or someone who supports "transgenders"?
(I'm not sure which is worse: being a transgender, or part of the community of transgenders! Personally, I think of myself as transgendered. It's a concept that has the benefit of actually being defined, not to mention definable.)
Anyway, we have transsexuality. And we have transgenderism. ... What is transgenderism? What on Earth is transgenderism? I haven't a clue - you'd be best asking one of those goonish writers whom impart favor upon the term. We have transgender and transgendered. We don't have "transsexualed". "Ooh, the poor soul, she was all transsexualed, she were!" (Say it in an Eric Idle voice... It almost (!) works... :-) )
I guess this here transgender needs to hit the horizontal. Small wonder the people who advocate against -ed do so. If they could write, they'd not be so perishably against the suffix.