It's totally normal to be worried about your future, I bet you did when you thought you we a "normal" couple. I hesitate to say that it's not supportive to tell your "Stop, wait, what about me?" as much as it a need to discuss how things are going to be with some certainty of what your future together is going to entail. I know you probably all hate me because I got to choose my partner knowing she was transgender and am therefore a happier person for it. I haven't invested all those years in a relationship to be surprised in such a way. But I am real good at seeing myself in another's position and the way I see it is that a transsexual has spent their entire life playing the role in society that their body places them in, including with you. Now it has become unbearable to them to continue on that way, they NEED to make their body match their mind, and they have about as much choice about it as you do. Well, you also have to do for you. If you absolutely know that you can't be in that kind of relationship, if you simply cannot be romatically involved with someone of the same sex physically, know matter how much your brain loves them, than that is something that you also don't have a choice on. See, if I were in your shoes, I would be okay with everything (after the initial shock, that is) because I'm okay with everything right now. But if I wasn't bisexual, it would be a big problem for me. The thing is, nobody really thinks about the possibility of their hetero spouse actually ending up being the same gender as them. And why would anybody? Chances are, if you're not bisexual and you knew what was going to happen before you got involved with the person, you probably wouldn't have gotten involved with them. (If I'm way off on this, please tell me!) If you find that you cannot continue to be in a committed relationship once the transition happens, it's not really that you're not being supportive, you can't help what your sexual preference is. But you can't really ask them to not do it, either. Because they can't help being what they are (and if I were trapped in a man's body I would get right on that transition thing!) So I guess it's really a fine line in terms of what is supportive, what is a neccessity for your own well-being, and what's not supportive. Just my 2 cents!
~angelsgirl