I agree that regardless of if it's socially acceptable to remain friends with exes or not (which I tend to do but I conceive it isn't for everyone and it really depends on the type of exes too), op you can go with the option you find yourself comfortable with. Your current gf sounds a bit insensitive and while there isn't anything inherently wrong with talking about her past sex life (depending on the tone it can even be something quite positive in the sense it does manifest trust towards you), it's also understandable that it triggers you. Depending on your background and where you want to go and do with that person, this aspect can be negotiated or not. None of you is wrong or right, you just need to find the right accommodation for everyone. If boasting about her exes sets you off, that's a conversation you need to have.
Personally I had a context where a partner of mine turned abusive and violent because I told funny stories about my sex life. I was in trust and just found sweet to share, and it really came from a light hearted place. Never I could conceive he would develop a terror of me abandoning him for someone else as I'm pretty much as stupidly loyal as a swan. And he became coercive and abusive towards me. And I don't mean you're doing this with your partner at all, but the line between explaining something is hurting you because of your personal context and that therefore you need extra reassurance on trust, and ending up blaming the person for the lack of trust in the relationship (internally or externally, again I'm not assuming you're being abusive or coercive in any way).
I might be a bit triggered myself with this thread because my ex had PTSD and also was paranoid about cheating and conflated cheating with the rest as the cause of PTSD. Obviously things mix with trust issues that are massive with PTSD and the risk of cheating are physiologically real, too. And not only HIV. What you went through is frankly atrocious and it really deserves to be considered by your partner. In my view this is a team working. It's not only your issue. It's your issue in the sense you're the sufferer of the disorder but as your partner it's also her job to hear you and accommodate you in a reassuring and loving way. Not by cancelling triggers but by showing you that even if she speaks of her sexual past, it doesn't mean it will result in what you fear. Both are in fact totally disconnected.
I hope you're okay and in a good place. These questions really are painful and difficult. Gentle hugs, if you accept.