I agree it is a difficult subject to handle; and should not be taken lightly. Although it depends on both people in question, it is probably a most difficult path faced by transgirls that date cisgendered men (like myself).
My general policy is that I will only tell those whom desire a serious relationship with me, but even so, it is extremely hard in doing that. As I mentioned in my introductory thread here, I am a pretty emotional person, beneath my cold exterior. A lot of times I start to cry when I tell that kind of information to him, simply because I am afraid that the person I like will no longer like me back because of some sort of misunderstanding on the matter. Despite the fact that it has very much hurt me in the past, in that quite a few people are not only not understanding of my 'issue', but are just plain hateful when it is not necessary.
(Sorry if the following derails the thread by the way, but I find it on-topic somewhat, and is just a personal story of mine.)
The worst case i can think of, which was pretty recent actually, was that a friend of mine (whom we tried a relationship before but didn't work out well and for long, but he claimed it wasn't because of my condition) tried to hook me up with one of his close friends. I got to know him somewhat well, and he seemed like an overall nice person, eventually he said that he liked me a lot and wanted to take it further. I'm a shy person overall and I just want to help others, so I decided to tell him about myself via messenger, and saying it didn't go over well would be an understatement. I've never seen someone so upset, he said I made him gag and have to throw up before blocking me, and then we talked through my friend the next day he said so many mean things to me until I had to tell my friend to stop relaying the messages because I was hurt. I wouldn't talk to him for a few weeks until my friend said I should send him a message on facebook apologizing. I did so, and he just replied with a silly stupid picture and blocked me there. 1 month later he unblocked me and accused me of hacking his computer, which is something I would never do, let alone if i even knew how. I then had to block him on facebook because he continued to harass me, and what's more is that he claimed 'I' was obsessed with him...
Anyway, enough about my personal misfortunes. I agree with Octavianus, in that you should not let it get you down, but continue on your own path.