That's the part I don't really get. You were born a guy but realized you're actually a girl; to simplify it. Why would you not want someone accepting of this and prefer that you had to hide it? (Okay, okay, I know why.)
How do you hide it? I can only imagine any "reveal" later is far worse. Most people are going to think you lied to them. It sucks, but thems the breaks, it's like that thread about the whore wife who tells the dude 20 years later. It's more about the hiding something than it is about the actual secret. (If you assume the person actually does love you.)
I could see waiting a little bit, couple dates or whatever, but it does seem like a pretty significant trust issue.