Because humans have an incredible ability to program (or de-program) themselves to a huge extent not really appreciated by most. Maybe the ability to do that is an adaptability trait or something, but people do have the ability to 'alter' their programming as it were.
Case in point: false memories for instance. There have been cases where individuals simply construct detailed memories of events....that never happened....and yet, they would swear they were true, pass a lie detector test in regards to it, etc.
Not that I am saying the false memory example is a good one because of the choice element (or lack thereof), but I merely offer it as an example of what is indeed possible. I think it absolutely possible for someone to become asexual by choice, just like I think it can be accomplished for any sexual preference.
The human mind is a truly amazing thing, and although we know a huge amount of data in regards to it, there is still a whole lot we dont know. And no, I am no expert in this, but this opinion is gleaned from helping my wife study for her advanced psychology courses for her masters, and just reading over some of the material covering this kind of stuff in helping her prepare for tests.