I think a good general rule is -- if it's not yours then don't sleep with it. He obviously left you for someone else, so it doesn't sound like he's yours anymore. Move on and stop sleeping with him. Being the father doesn't give him the automatic right to have sex with you. HE should continue to be a father, and YOU should stop letting him be player.
He is the Father of your child. You will never be able to cut all contact with him. But, sleeping g with him if you do not love him is stupid. So, he left you for another girl but, did not sleep with her? So, what did they do together? Bake cookies? Come on, he slept with her and you want him back. You share a special bond. You love this guy and sleeping with him will not make him get back with you. Now, if you want to get back together then, work on a healthy relationship with him for you and your baby. He isn't just a sperm donor.