This is the whole intrinsic/environment debate as far as I'm concerned. From my experience with talking with lots of different people I've concluded part of your sexuality is who you are as you were born, and the other part is by choice. It is similar to the genetic/environment debate. From person to person it varies and so people will always argue about it.
I used to think Bisexuals were just crazy, but I'm now convinced they are the most liberated. I'm not bisexual, but I've come to know human sexuality enough to come to the conclusion part of it is based on an innate attraction, and the other is from what you have come to the conclusion of with respect to your own sexuality and how sexuality is defined by others around you.