Whoa there nomad! No one accused you of anything. Take a breath man.
Lots of women misrepresent themselves in lots of different ways, so do men. Its part of the game. You seem sure of her being trans, but think of this woman's situation. She see's herself as a woman, and has had herself modified to look and feel the part. She may not consider herself to be a man masquerading as a woman, but rather a woman who was previously trapped in a man's body.
So when does someone in this position disclose her status to a date or sex partner? Online? First date? Before hitting the sheets? After? At all? Thats a personal decision no one else can answer for her. This is uncharted territory.
At the same time, you have a right to your feelings about this. I would say in general that a CSA/ASA survivor would be especially sensitive about this because of our history of being betrayed around sex. Finding out you were rolling around with a genetically male sex partner certainly could be very triggering. How's a guy supposed to feel about this? Some men, like your buddy, have no problem with it. Others would. Again, this is uncharted territory.
If I were in your situation, I think I just would end the relationship without confronting her about her gender identity, and then try to let go of it. Learning where NOT
to put your penis is a part of being a man. Really. Consider this a lesson learned, and move on.
BTW, there's an old song by the Kinks about this situation, called "Lola" Here's the link: http://youtu.be/Ixqbc7X2NQY"Girls will be boys and boys will be girls
It's a mixed up muddled up shook up world except for Lola"