Your body belongs to you. Nobody, no matter who they are, if you know them or you don’t, if you’re in a relationship with them or you’re not, ever has the right to do anything to your body that you don’t want to happen.
When it comes to sexual things only you get to choose what you do and what you don’t do. It’s also totally ok to change your mind about a choice you’ve made too, at any time. This means that if you’ve done something sexual with someone before, you don’t have to do anything with them again.
If someone ignores the choice you’ve made, or if you can’t make a choice you haven’t consented to something sexual. Sex without consent is rape. Other sexual things without consent is sexual assault. Both of these things are not ok, both are crimes and there is help out there.