It sounds to me like your dad has very cut-and-dry rules on what is and isn't gay. What is your relationship like with your dad? Does he have problems with gay people, or does he just think that you're doing wrong by dating a girl when he thinks you are gay? It may be worth trying to sit down with him and fully address the fact that you're heterosexual. Try to relate to him and ask him to tell you some things about his experiences as a straight male, like who he first dated or what he looked for in women. It sounds weird, I know, but he may need to see you less as his son who he's responsible for and more as a friend, which will help him understand who you really are.
