I used to consider myself 100% gay because I was physically and romantically attracted to men and had no interest in women. However, as I got older, I started getting curious about women - more specifically, I started wondering if I had been "missing anything" by not having slept with a woman. At first, it was confusing to say the least ("Wait! I'm gay! How can I be thinking that?"), but eventually - many years later - I've come to the conclusion that it's not "just a phase" and that there's something more to it. But then it gets weird...
I'm still physically and romantically attracted to men - nothing's changed there at all; what's changed is that now I'm also interested in having sex with women. Oddly enough, I don't find myself attracted to women physically (I did say it gets weird) and I have no interest in relationships with women beyond friendship (possibly with benefits). It's not that I'm turned off by women's bodies or by the thought of a romantic relationship with a woman, it's just that I don't find myself thinking of women "in that way" - unlike men, where I'm constantly thinking of them "in that way"!
I think a more accurate percentage system would use two independent numbers, one to measure your attraction to the same gender and one to measure your attraction to the opposite gender. The current system treats gay/straight as an either/or proposition - if "gay" goes up, "straight" goes down, and vice versa - whereas the other system would allow more flexibility. Under the more accurate system, I would have to say that I'm 100% gay (full physical and romantic attraction to men), and also maybe about 30 - 40% straight (interested in women sexually but not romantically).