I guess I'd talk to her about it and try to figure out why she feels that way. In the end, though, it would have to be her decision who she is going to be. I can try to guide her, but i can't get into her head or make those sorts of decisions for her. I admit that it would be hard for me, though. But then I've never had gender issues.
I'd think she's been watching football with her dad too much.
Let her be who she is. You cannot and must not try to form a child into something their not. Do you want your child to have a happy life or one full of resentment and hate because you didn't support them. Tell her you will always love her, (and here is the most important part) unconditionally. You also need to educate yourself about the transgendered. They are not evil one eyed monsters from planet X. They are someones, son, daughter, brother, sister. They need love, not hatred because they are different. Did you decide one day to be who you are, our did you just know?
Honestly, I'd feel a bit sad because I'd know that she had some difficulties ahead, and I'd have my own mourning to do of the loss of whatever fantasies I'd had for her life. But the bottom line would be that this is my child's life, and I would have to be supportive and help her (him) in whatever way was needed.
you should WantHer2BeHappy
Of course I'd want her to be a girl. But it's her life, and I'd love her no matter what gender she chose. So, I guess I'd accept it, and learn more so I could understand better.
If he/she is transgender and you didn't know it, I'd have to think you to stupid to have children in the first place. But then again, I just looked back at your handle - you've got a T there. Correct me if I'm wrong, but doesn't that mean you are transgender? In which case the only way you could have a kid would be to adopt, and once again - even in an adoption, you would have to know in advance if you are adopting a male or a female. In short their would be no reaction because the question you ask could not take place in reality.