I feel the same way, despite my SO telling me that she is not transsexual and will not transition.
... the trouble is, there are so many people who say this but then change their minds down the road. Also, I have seen her grow in her expression so much since we met, and logic dictates that if there has been a pattern of growth, there might continue to be growth? For example, my SO once said that she didn't understand why crossdressers would want to wear a bathing suit. But several years later she bought a bathing suit with the intention of going swimming at a LGBT friendly place in our neck of the woods. I fully support my SO if she wants to do this, but I directly observed an instance where she did change her mind about the depth of her expression.
Overall though, I don't spend a great deal of time thinking about it and when the thoughts do cross my mind, I tell myself to focus on the here and now and not on what may or may not happen in the future. Besides, I've no idea how I might react should she eventually realize that she is not happy living as a male. I cannot say that I would reject it because I have not been faced with the situation to determine this. But, I do need to be honest and say that the thought does cross my mind occasionally. I need to stress though, that it is only occasionally.