After reading this board for a year, I think of CDing as a progressive disease, but with no way to tell how fast or how far the progression will go. And I calm myself down by thinking: suppose the worst does happen -- he eventually gets a very serious form of the condition, and replaces his current persona with a different, female persona. Would I want to have spent the previous years with him? Of course. I treasure the days/weeks and hopefully years we have together. I don't base my happiness on our future growing old together; I base my happiness on the fun times we have together right now.
In the future, if he gets depressed, then we'll have to see what happens then. Maybe I'll feel lucky that we have tools to help him out of the depression (aka HRT), and maybe I'll appreciate his happy female persona more for having spent time with his depressed male persona. But for the moment, I enjoy the time I have with the happy male persona I love and I try not to borrow trouble from a future none of us can predict.