I think people very early find out that there's something "wrong" with them, they may not be able to put a name to it, but they feel it. Often, not always, they'll find ways to express their true gender even if they don't realize they're doing it, much like many gays and lesbians do when they are in the closet. What I'm saying is that even if someone doesn't "figure out" what their true gender is, I don't think its something that anyone paying attention to their partner wouldn't notice in some way, even if they don't acknowledge it, much in the same way, some spouses in heterosexual relationships might see signs their partner is actually gay but may not know how to recognize it or name it. It's not like someone out of the blue starts acting like a different gender. So that's why I think, even if someone was transgendered and didn't know it, I think people would pick up on this confusion even before they started dating and either it wouldn't come as a surprise when their partner told them the truth or they would be comfortable with the idea.
Another wrinkle, what if your partner realizes they are transgender and then realizes they are still gay and you're incompatible with them? That also happens in a few cases because gender and sexuality aren't as tightly linked as one may think. Would you want your partner to dump you because now you're the wrong gender?