If you believe in other genders and are attracted to either biological sex, it'd make bisexuality an obsolete term, hence being pansexual.
Personally, I still have a hard time taking the gender fluid stuff seriously. It just reeks of "hey, look at me, I'm different!". I guess that's the point? That they feel different enough that our two gender social system doesn't fit their lifestyle? You'd have to be pretty left field, even contrarian, to even consider it. That, or just riding a trend.
There's been people who toe the line of gender for ages. Maybe these type of people felt so ostracized for their eccentricities - dabbling outside of norms - that they were like, fuck it, gender is a social construct anyway, let's just make a whole new category for ourselves and see to it that future generations acknowledge it as normal.