Most trans people do not use "transgendered". Nearly every person who posts a thread in r/asktransgender which contains that word gets a polite correction, as it just isn't correct.

Words are defined by useage. Tons of people use "transgendered". Even tons of trans people. It's not bigoted, it's not ungrammatical. When I type it on my computer, no little red line pops up. It's equally as correct as "transgender", and in my and many other peoples' opinion, it's more natural sounding, which is why so many people say it that way even if they have never heard it said that way before.

The use of the word "transgendered" seems to have come about due to misinformed media sources who are now starting to tell our stories. One person sees an article with that word and assumes that the word is correct, but it isn't, it's just that the author didn't do their homework.

That's just not the case. Have you ever heard of the wug test? It's a test where children are asked to pluralize the non-word "wug", and it demonstrates that even if a child has never heard a word before, they are able to apply the same grammar patterns to it that we apply to other words. The same is true with "transgendered". "Transgender" is a word made up of the noun "gender" and the modifying prefix "trans". In English, when nouns are used as adjectives, they often take on suffixes or particles that make them sound more like other adjectival forms. People don't need to hear other people say "transgendered" in order to naturally have it come out of their mouth. It's just an application of normal english grammar.

Here's the biggest problem - we, as transgender people, are really fighting for acceptance right now. We are at a tipping point with [American] society and so we are being very careful not to upset anyone who is ultimately supportive of us. Acceptance is the main goal and so arguing about words can seem quite pedantic. I really don't think that's fair though. I don't think that, just because we really want people to accept us, we should have to concede the language we use.

You shouldn't. Nobody is telling you what language to use, and in return, we're asking you do the same. Use whatever language you want. Words change over time. Maybe in the future "transgender" will become the norm, and then maybe it will change to something else. I'm not telling you that you're wrong. There's nothing wrong with your usage, and there's nothing wrong with mine either, so stop telling people that they are wrong.