@Tony Johnston: Because of the simple fact that the only people who can be sure of their HIV status, as far as sex goes, are virgins and PwHIV. You can’t prove a negative. All a negative result means is that up to three months ago you didn’t show HIV antibodies in your blood. You can get a negative test result during the window period, before antibodies have formed, or you can pick up HIV the very next day or up to three months later. Negative test results are not reliable.
As a positive man I prefer to limit my sexual partners to guys with HIV for fear that someone who thought they were negative suddenly gets a positive test result. There are strong legal implications involved. Personally I don’t see that the law has anything to do with who I have sex with, but it seems people who think they’ve negative need to be protected by the law and the perceived positive partner needs to be punished. In other words we, the HIV+, are made responsible for your, the HIV-, safety. If you say “let’s not bother with condoms tonight” and you get HIV it’s our fault.
The guy most likely to give you HIV is the guy who thinks he’s HIV- but is actually HIV+. In the UK we know we have about 100,000 PwHIV of whom only 76,000 are known (UK law allows for anonymised testing of blood samples). That, in itself, is a very good reason for avoiding the “I’m neg, UB2” culture.