I'm neg and on PrEP. Not trying to make this a Backroom post but I love hearing about my partners' prior experiences. In a different vein, I've asked a "straight" married friend if he remembered the specific session when he impregnated his wife. I frequently ask gay friends about their early experiences, etc. So yes, when I'm with a poz guy I wonder if he knows the guy that pozzed him. To know is just another creepy turn on for me, not judgemental, but I do hesitate to ask because I don't want to offend someone (especially before we've had sex.)
