I [34F] caught my partner [42M] talking with an AI Sex Bot

This just happened and I am still a bit shaky and not sure what to do. For background, me and my partner have always got on fine. We are very sexually vanilla and have never had any real issues in the bedroom (or at least i thought).
Today I logged onto his computer to check my email and I saw a website up with images of girls. I thought it was just porn or something, but when I clicked it was one of those new AI / ChatGPT sex websites where it calls you and has a conversation. I had his woman "Alana" call me just to see, and when she picked up she greeted me by my partner's name and then asked me if I was "ready to be dominated." I asked it "what do i like" to see what it would say and it responded with an explicit description of BDSM that I wont write out. It sounded grossly real and it was clear he bought at least a few "sessions".
I feel confused and betrayed. I'm okay with porn, but I feel like we have always been very open with each other and the things it was talking were disgusting to me. It feels like my partner has a second life. How do I bring this up to him? I feel uncomfortable with this fetish and I don't know how to have a conversation about this, especially because I don't know if this counts as porn or something more sinister?
TLDR: caught my partner talking to an AI phone sex service