Well, it's tough one eh? First off I guess it depends whether or not the STD is something that you can get cleared up.
If it's not, my inclination would be to avoid sexual contact as long as you can. Even with condoms, my thoughts are that if things progressed and you finally told her, it would certainly be something she would be upset about (or even angry) that she didn't know *before* you slept with her. After all, you know all about your STD, but (ladies correct me if I'm wrong) she would want to be well informed of the risks before sleeping with you. I would imagine she would take your lack of communication as arrogant and selfish.
Get to know her, and let her get to know you better. If you feel it is something special and could bloom, what's the rush with sex? If it gets to a point that it really looks like it is going to florish, and she is confused about your reluctance to sleep with her, maybe that could be a good time to talk to her about it.
I think your statement is probably true,...if you're talking about meeting someone and telling them on the first date! HOWEVER, if that person has feelings for you, I would say it changes things dramatically. Noone can switch off their feelings, and although some would still walk away, I'm sure a great deal more would just need to be educated on the subject to be able to understand.
I hope this helps mate,