Why should someone post op have to?
I take a crack at that. I think its because when you fall in love with someone and want to spend the rest of your life with them you are buying a total package, not just the out days. Most people like their lovers past to a degree. They want to go back and find out about things they shared, did in common, and both relate to - the want to know they came from the same place to a degree. My GF sometimes asks me to tell her a story from when I was little. What exactly am I going to say? Because if not the truth, the other option is pretty much a full cloth invention.
Let's just assume here for a moment that I'm an FtM like Mister. So I'm telling stories about that Little League game where I did a pretty good job pitching, or building the soapbox racer with my dad, or how my friends and I had hid a couple of Playboy mags out behind the garage, about working on cars, doing stupid little macho tricks (when I was in HS you were not 'one of the - really stupid - guys' until you jumped from the Hacienda Bridge, about 40 feet above 15 feet of water). You know, all that stupid guy stuff. And there were the awkward first dates, the even more awkward first attempts at sex with Suzie in the back of my Ford, and other growing up stuff.
Yet, none of it's true, far from having the 'happy normal boyhood' I'm talking about above, none of it happened, and in reality I was a very miserable little girl. That is a huge difference - that coming at something like life and relationships from a perspective of having a happy or miserable childhood.
In fact, if its not how you see it, then it's for sure how the other person will see it when they find out - they fell in love with a lie, not a real person. A fabrication, not a reality.
How do you explain how it is that you don't have one single photo, report card or anything else from your childhood? Why there is not a single person around who knew you when?
I'm not sure that on rare occasions that some guys might even go along with a TS GF, but when they find out one of the things they are finding out is, in the immortal words of Sam Kinnison, that you're not just a TS, you're also a 'lying little bitch.' And all guys have had at least one girl like that, and knowing even just one is way too much as it is. Add to that, the persistent and nagging feeling "What else are they not telling me?" that's going to haunt them for the rest of the relationship with you.
There are no shortage of posts in here that talk - at length - about the need for communication, honesty and sharing as part of a good relationship, and no matter how good you are at it, its all just a pose, its never a reality, so, in reality, you're relationship has none of those things in it.
I can see where that can be disappointing when you find it out. It's hardly a reason for murder or violence, but it's all the reason one would need to hit the road and never look back.