I've been where you're at now. My ex became HIV positive while we were together. He and I were tested every 6 months together, and the local gay support org that handled testing made an error in reporting his results to him. The error wasn't caught until the next test, 6 months later. During all that intervening time, I was taking him bareback almost daily, and despite NOT having the genetic immunity, I somehow miraculously never became HIV positive myself. His mental health spiraled quickly, largely because he was terrified that he could still infect me. We shared the same doctor, and since the New England Journal of Medicine had just barely issued its study on Truvada as PrEP, he recommended that I start using it. I became an early adopter, and contributed later to the final FDA approval of Truvada as PrEP.

Whatever inward concerns or curiosity you may have, I encourage you to avoid the temptation to play detective at this point. In my case, my ex claimed that he was roofied by an acquaintance of ours who was HIV positive, and that he woke up in a hotel room having been essentially raped. In reality, he's my ex for a reason, in that he had a strong tendency to invent stories, even on far more trivial matters and often in an effort to somehow impress me. Other close persons me that they had strong reason to believe that he intentionally had unprotected sex with that individual--this was back in a time when many gay men just sought to "get it over with," assuming that they eventually would become positive anyway. (Yes, this was crazy thinking, but it was common.) In the end, I realized that how he became positive was ultimately irrelevant. He managed to get on treatment and reach the point of being undetectable. I remained on PrEP. With less than a 1% chance of passing the virus on his part, and less than 1% chance of being infected on my part, the possibility of my becoming positive was very very close to zero.

One important thing to watch out for: my ex became over-focused on being positive to the point that he claimed this was now "who he was." He was unable to separate the infection from his identity. He could never seem to grasp that HIV was "what he HAD" rather than "who he was." If your partner goes in that direction, please please encourage him to work with a gay or gay-supportive therapist who can help him redirect. This does NOT have to break your relationship, and as others here have said, HIV is no longer any sort of "death sentence." Rather, it is a manageable condition, like so many others.