Just google Polyamorous flag xD
Long story: It was originally chosen because the mathematical constant π is an irrational number with infinitely many decimal places, a play on polyamorous people "having infinite love". Another use of π was as a furtive shibboleth, a symbol whose meaning would not be understood by non-polyamorous people, to allow polyamorous people to recognize one other without the risk of being outed or exposed about their non-traditional lifestyle
Where's the american flag?