Well, technically you were dating him first and she didn't respect that so I don't see where one could really throw rocks at you about continuing to sleep with him.
However, you need to be your own rock in your life. Especially with an abusive childhood. Leaning on others will repeatedly develop codependent relationships for you, and that's a really not good thing.
Stand up for yourself, you've done the right thing, and maybe look into support groups/therapy to help you cope and become your own rock in your own life.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."