AITA for telling the other woman that her boyfriend is having sex with me?

AITA for telling the other woman I exist? It came to my attention that the man I was casually involved with for two years is seeing someone else. When he reached out to hang out I asked about this other person. He said she was someone special but on the other hand I am the best thing that has ever happened to him. He begged me not to tell her as we just hung out and had sex the day before I found out. I told him I will not continue with this and he keeps begging me not to tell her. I would want to know because I found out he does this all the time. Advice welcome.