On another forum someone suggested 'pan' acknowledged the infinite possibilities in gender and sexuality rather than having to conform to binary models. And furthermore being 'pan' was a declaration of ability to love(/be sexually attracted to) anyone anywhere in this infinite spectrum.
That led to an interesting philosophical discussion that if there is an infinite spectrum then it is impossible to declare such an ability, as you could never know if somewhere within that infinite range there was a combination that wouldn't be attractive to you. The mental gymnastics quickly became exhausting. 
It wouldn't be so mentally exhausting if you turned the definition around. A pansexual person just wouldn't rule anyone out as a partner based on their gender or sexuality, so they wouldn't need to acknowledge an infinite number of combinations that they could be attracted to. Of course, like anyone else, they are attracted to, or repelled by certain people based on all sorts of other attributes - sense of humour, intelligence, degree of extrovertness, appearance etc.
