Mom's listening to and repeating part of evil Christendom and Dad's got a problem with porn and YOU are the bad guy here? She is going back to him and living in the family home? doesn't matter where, the home is half hers if they are married she can have anybody she wants to come and visit, including you. He will be annoyed and bluster but after time he will get used to it. She can let him know ahead of time if she cares to and he can LEAVE.
You can let her know your door is always open and would love to have both of them in your life but having a clandestine relationship with your own mother is rather stressful, in the future it will be phone calls or in person visits and if dad doesn't like it too bad. If you continue to email the both of you will only be enabling your father's bad unchristian behavior. Let him get out of his comfort zone, he gets mad? so what.