@Purple I’m sorry you’re going through this! Not having control over people’s perception of you is really anxiety-inducing. I had an ex tell lots of people about my sex life while I was in high school, so I can really empathise.
When I was going through that horrible time, I kept trying to think about how little space this rumour about you occupies in other people’s heads. Humans are so self-centered and anxious about their own stuff, that this rumour will be a blip in their head compared to the huge amount of time/emotional energy you’re expending on this. Reassuring yourself of this can really help with the anxiety of it. And as much as I wanted to confront my ex about it, I just avoided him and let the rumour die down. Even if people had this false idea about me, I came to peace with it knowing that another rumour/hot goss would succeed it.
And @EarthMama is totally spot on. The rumour spreading says more about him than it does about the rumour itself!