If you're in a sex-less relationship, is hooking up with a former sex partner OK for meaningless sex?

My girlfriend is used to being single, and now were together. For personnel reasons we haven’t had sex yet. She assures me she is being completely faithful to me, but I found some very disturbing things lying around suggesting she is seeing other men for meaningless sex. I don’t know what to make of this. Is it OK? Even if she’s not telling me? She knows i get extremely jealous of other men, even if they’re just friends. But I honestly have no idea how to address this situation.