Hmm....It would REALLY depend...
*Is a guy*
If the person used to be a guy, but changed to become a girl by the time we started dating, I'd be ok with it. Yeah, it would blow my mind and I'd need to think about it, but I'd probably get over that pretty quick.
As for the other way around, it's a bit more complex.
If I knew they were trans (while still female), then I don't think I'd date them.
If they were female, and then eventually decided to change to male while i was dating them...I don't know. I have no clue what I would do.
FYI, though, I know a FTM trans person who's actually pretty damn cool...And they have a fiance (male), that was dating them LONG before they came out as trans. They're still together and doing fantastic. I applaud them for being able to stick together despite the change.