I'm so sorry to hear you're having a hard time. Even with my own supporting family I've had to deal with similar discussions. My mother simply assumed that my boyfriend must be gay. I've had to explain that he is straight and is simply into girls. She went on to mention the obvious fact that I still have a penis, I just went, "and?"... Over the course of the long conversation I helped her realize how he can be straight and still be my boyfriend.
I've also struggled with the idea of, "maybe I'm just gay", when I first came to terms a couple of years ago. After lots of self-questioning, and long, sleepless, thought-filled nights, I am now sure that I am not "just gay". It just doesn't fit. Although I believe my sexual orientation and my gender identity have nothing to do with each other, the line between them can still be blurred.
A lot of people tend to think that we transition with the goal of sex in mind. And while that may be a goal, it's not the only one, and it's certainly not the main one. We just need to exhaustively explain the difference. It gets tiresome, but the payoff is worth it.