I fantasize about watching my boyfriend fuck his ex. He used to cheat on me with his ex all the time. Why am I aroused by something that is painful for me?
Okay, you’re not fantasizing about your boyfriend fucking his ex. You’re fantasizing about watching them fuck. That’s a key distinction. This is still very much a sexual fantasy about you and your boyfriend, but it’s about humiliation.
Instead of getting off to the thought of having sex with your boyfriend, you’re getting off to the thought of being sexually humiliated by your boyfriend. His ex is only important in the fantasy to the extent that she is semiotically charged and represents that sexual humiliation.
Don’t freak out about this. Humiliation is a pretty common fantasy. There’s even a term for a woman who gets off on being humiliated by her boyfriend having sex with other women: a cuckquean. Please understand, I’m not name-calling here. If that’s your thing, cool. I’m guessing at this point, you’re still trying to figure it all out. That’s fine.
I will say this though, the fantasy is undoubtedly your unconscious mind continuing to process the emotional pain of your boyfriend’s infidelity. Even though you may have outwardly forgiven him, you’re not over it yet, and the fantasy represents some sort of defense mechanism at work.