My girlfriend and I are on a break. I don't want her dating anyone in the interim.
Being on break means she's free to do whatever, though, right? Why can't she date?
Because if she tries to date other guys, I'll break up with her
It only seems logical your ex would be available to date others while you're currently not dating. Why not just do a complete break up?
My ex is a slut.
Listen, as unfortunate as it sounds, you don't have the right to make ultimatums about your ex's dating life.