I've had similar situations where I've revealed I'm a transsexual, and have been asked, "so, you're becoming a male?"
People don't understand. We're deep in the middle of the phenomenon (we ARE the phenomenon,) and we know so much about ->-bleeped-<-, because it's a part of us that doesn't really go away. But we're somewhere between .04% and .5% of the population (Lynn Conway numbers,) and most cisgendered folks have no idea...
In such situations, I take a deep breath and look at it as an opportunity to teach.
Karen