Honesty is really an essential part of life really.
But I think it was Ben Franklin who drew a distinction between honesty and not saying what isn't strickly relevant.
There will come a time, with any of our relationships, when we need to give information that is relevant. But I can't see why we need to say things about ourselves, which aren't. When you first meet someone, you don't know everything about them. As you get to know them, you learn more, but a little mystery keeps relationships interesting.
As others have said though, as for your medical history, when it becomes relevant, you can discuss it. If someone, after getting to know you as an individual and developing a relationship with you, changes because of something that is past, then they probably weren't worth it in the first place.
kyril makes reference to an interpersonal relationship. This is a good case in point. If your relationship has deveoped to the extent that you feel comfortable about being intimate, then what you have is surely superficial.
In the case of casual sex, what you have is quite important. But equally, the whole point about casual sex is the pot luck element. Sometimes, you pull out a bar of chocolate, sometimes a steak dinner. But if you're not ready to take the chance, (the other person as well of course), then don't try the pot luck.
I really do understand your position SadGirl. I might have the same approach, in a similar situation. Though it seems to me that we have nothing to apologise for, no reason to ask for permission. I will keep those aspects of my life private that I choose to, because they are my business. But I have no reason to feel any shame for being me.