This reminds me of a friend who had been adamant all her life that she was "100% straight". She even found homosexuality a little creepy. Then all of a sudden for reasons she couldn't even explain herself she found herself attracted to another woman and eventually they embarked upon a relationship. She had never, and to my knowledge has not since, been interested in women except for this one - saying only that there was "just something about her" that drew them together.
That's only one case of course, but my point is, can anyone really state with absolute assuredness that they are 100% straight, unless they happen to have met every single member of the same sex on the entire planet and not been attracted to any of them? Attraction is an extremely mysterious thing and can hit one square in the face, sometimes coming as quite a shock. It's unpredictable and I think saying you're 100% straight is like saying you'll never go to the cinema because there will never be a film you'll enjoy.