Going to attempt to look at it from both sides...
If i would have the virus, I'd probably be torn between telling and not telling. If it was a person i haven't met before, i'd probably hold off until i got a sense of where things are going. If it was someone who i've known for some time already and started to get romantic feelings for i would probably be quicker to tell her. As long as i haven't told her i carry the virus i would feel guilty about it. At the same time, i'd be scared to tell because of the negative reaction i could get.
If i would be dating a girl who had the virus and i have no knowledge of it, i would want to know about it as soon as possible. I'm not sure how i would react, but i'd probably be scared initially, maybe enough to not date her again. Not just because it could affect my own health, but also could lead to getting my feelings hurt. I honestly don't know if i would be able to overcome my initial reaction. I hope i would, but i might not. I would probably get quite upset if she took 'too long' or things 'too far' before telling me, possibly resulting in me not wanting to ever see her again. Tough either way really. Not sure what 'the right moment' would be if i'm perfectly honest.
This is how i look at it right now. I know it's hypocritical of me, but that's how i feel about it when i'm being honest. I might react completely different if i would actually be in either situation.
If a friend or family member would tell me he/she has HIV, i wouldn't turn and run away. I know very well HIV is highly managable these days, not the death sentence it used to be. You don't get HIV from giving a hug, or lending a shoulder. I've heard reports of some people being immune, and i think recently there was a case where someone actually got rid of the virus. I don't think it will take much longer than 20 or 30 years before a working cure for HIV will be on the market. By then, this will be a non-issue.