Your wife is saying what most every wife says in the beginning, except it usually comes in the form of the question, "Do you want to be a woman?"
I would say that a huge chunk of GGs cannot see themselves married to spouses who have transitioned from male to female. So your wife just saw Bruce in the media, and she wants reassurance, that's all. Just keep doing what you're doing ... tell your wife that you are a man, you will not transition, and you can add that the vast majority of CDers are like you. There aren't a lot of Bruce Jenners out there.
For what it's worth, I kept asking my SO if s/he was going to transition for years! It takes a loooooong time for most people to understand the difference between CDers and transsexuals.