Well, I tried to vote that it's annoying, but I accidentally tapped that it is a stain as I was tapping submit.
Anyway, I find it roughly equivalent to fingernails on a chalk board. It's grating, but not insulting or offensive per se. But it poses problems aside from just being grammatically equivalent to saying someone was "gayed" or "talled."
For one thing, it sounds like the past tense of a verb. So when people end up talking about "transgendered" we also sometimes hear them talking about "transgendering." Pretty soon it gets pretty incomprehensible.
It also makes it sound like something that happened to a person rather than an integral part of who a person is. As in, "oh my gosh, I fell in the vat and got transgendered!"
Now transgender as a noun, as in referring to a person as "a transgender" or a group of people as "transgenders." That I find offensive, as do many people. I see it occasionally on this site and I refuse to respond to any thread where people are using it. If it is used that way in my proximity in person, the user will be corrected promptly.
The problem with "a transgender" isn't just grammatical. But take a look at what is written about us. Anti-transgender writers tend to use "a transgender" or "transgenders" instead of "transgender person" or "transgender people" because it allows them to avoid the word "person" or "people." It's a lot easier to defend hate, bigotry, and discrimination if you shove aside the notion that the recipient is a human being. I don't like having my humanity deleted and I don't like hearing my haters' language tossed at me.
