My mom thinks I'm gay?

I just came home from hanging out with my older sister when my mom called me into her room. I was kinda confused because she said she needed to talk to me and had a really stressed look on her face. I'm a pretty good kid and I knew I hadn't done anything to get in trouble, so I asked her what's wrong. Then she said, "I know about you and (your friend)" I was confused and then she elaborated. Basi cally she saw my facebook wall, where my friend (we're both girls) had written very sexual sarcastic jokes. I was quick to assure her that albeit innapropriate, the comments between us were innocent. The friend is actually on of my two roommates at the boarding school I attend and sexual jokes and banter is very common between teenagers there. However, my mother refuses to believe me, claiming that I'm lying and trying to make a fool of her when she knows the truth. She started talking about how disappointed she was and how I'm putting so much stress on her. Then she went on to say that she could believe I was disappointing God like that after all he'd blessed me with. I kept trying to explain the context of everything, but she absolutely refused to believe me. I ran out of the room crying after she threatened to tell my dad (who is a huge homophobe) and see how angry and disappointed he'd be of me. He actually made a comment last week saying that finding out one of his kids was gay would send him to his deathbed. But I'm NOT GAY! I even have a boyfriend of 4 months who she knows about. I'm crying like crazy and contemplating running away because she won't believe me and she's gonna ruin my relationship wit my dad and tear the whole family apart with this "secret" of hers that isn't even true. PLEASE HELP