Reading the posts over again, I think it is important to point out that choosing not to have sex with someone who is HIV positive is a choice that everyone must be able to make, and it is a choice that should be done knowingly and voluntarily, as with all sexual encounters.

It can be uncomfortable and embarrassing if two people are into each other then one says "I am HIV positive" and the other then has to make a choice as to how far they want to go. I think it is the responsibility of the person who is HIV positive to disclose that, just as I think the back-up is that everyone should ask with a new partner.
If one discloses that he is positive and you choose not to continue becoming more intimate, I would hope it is done politely, not "get away from me you damn dirty ape!" That person's feelings are important too, but those living with HIV also need to understand the apprehensions of HIV negative people in sexual encounters. HIV negative people should be understanding of the feelings of those with HIV, but those with HIV also need to understand the feelings of those of us that are negative.

Sex is so intimate that setting boundaries, whether they be rational or irrational, is always acceptable. While it is wrong to discriminate against someone in housing or employment for their race, religion or HIV status, I see nothing wrong if someone makes the personal choice only to have sex with HIV negative, thin, hispanic, jewish men between 30 and 35 years old who have a college degree, no substance abuse, no criminal history and know how to dance the macarena. Nobody else can tell me who I should have sex with, and I can set any boundaries or classifications I want (as long as they are over 18 and willing!). Nobody should ever feel ashamed to express their sexual preferences.