Yes, you can fight them very effectively. See, you aren't looking for a generic therapist, you are looking for one that help for Gender Identity Disorder, the diagnosis that will probably be billed. There won't be one on their list, I betcha, unless you are in an area with a special clinic. So, you will probably be able to push it through, if you have one picked out, by saying that they are specially qualified. You may have to talk with the company's medical director. Good luck, and persistence pays dealing with insurance!
Blessings, Tara