Ah being pigeon holed is a pain huh. I think lots of people think you should 'pick a team' and stick with it but (speaking as a pansexual person) it's not a very liberated view ;-) It is also not easy because most people don't get poly, so they scrabble for a model they understand. 'kinky straight couple brings in another girl' they understand. 'All three if us are in love', they do not comprehend and is more out of this world than most movies, so they probably subconsciously seek something more familiar to hang it on.
Says more about their minds than anyone else's though ;-) perverts!