Whatever works for you. I try to keep it simple in words even a non-poly-educated person would get. This is an "elevator speech", not a thesis.
"I am the hinge in a polyfi V. That means I'm the Shared Sweetie in a loving, committed, CLOSED relationship that is limited to just us 3 people."
Alternately "I'm the wife. I have two husbands. My legal husband is ___. My other husband is ____. We are happy together. "
Person you are talking to can have a cow or not.