Originally Posted by GalaGirl
The same way you know you are poly. You take the internal temperature of yourself and decide what framework you are going to play in.
Both mono- and poly- are relationship structures. Both are valid choices for an individual to make.
It's not biology.
I'm personally a huge fan of this particular answer. Biological makeup has nothing to do with your decision on what form of raltionship you choose to partake in. This is a choice made by self examination, feelings, and personal beliefs.