It is great that you are out and comfortable with your sexual orientation. It is normal for your dad to believe this is a phase and be in denial, but this can be very frustrating for you. It is likely that he may also be confused about what being pansexual means, so explain it to him, as much as he needs you to, and be patient with him. Also try being open and honest with him about who you are dating, instead of hiding it or keeping it secret. Over time exposure will help your father to understand, acknowledge, and accept your sexual identity.