That's huge. It's something I had to tell my mom too, but she was really cool about it. I feel bad for your situation. Maybe as you're saying it, you can explain that it isn't something you chose, because you would never choose to upset your family on purpose or have them shun you. I firmly believe nobody would actively choose to be anything but straight. It's human nature to want to be accepted, loved, respected. Coming out can destroy those things. It's just completely illogical for anyone to think it's a choice. Tell your dad you knew how he'd react and you'd never provoke him to be disgusted with you, that it's something you can't help or change, just like your gender or ethnicity. Tell him you'll be careful and that you're still the same person, just no longer keeping a debilitating secret. Good luck, I sincerely mean it.