Is it possible to have sex with an HIV positive person and not get it?
Yes, it is. First, the highest risk: even if you have the very highest risk kind of intercourse, unprotected receptive (you on the bottom) anal sex with someone with active, untreated HIV you still have a low chance of contracting the virus. Please don't play the odds - it's about one in 70 but you have no way of knowing whether you're gonna be that one. So don't chance it. 70 couples get laid in that scenario and only one person contracts HIV. I am including this only for completeness and transparency: it is not safe or responsible. Other kinds of penetrative sex have lower chances, but still. If the only kind of partnered sex available right now is totally unprotected, go jerk off instead. Or, hey, jerk off together. Just don't share any fluids. Second, use a barrier. If you fuck and there's a correctly applied condom, then unless the condom breaks or comes off, you will not contract HIV. Third, have sex with someone who's on treatment. If their viral load is so low - so well controlled - that it's undetectable, there are essentially too few copies of the virus floating around for any of them to have a chance of making it into your body. This is sometimes called TasP, Treatment As Prevention. Fourth, go onto PrEP. Pre-exposure prophylaxis. If you're on PrEP, correctly prescribed and taken, your body is totally inhospitable to the virus.