At the end of a 10 year M/M relationship am I repsonsible for a heloc loan, if I didn't sign for it?

We have two loans on the primary residence (both on mortgage) we're upside down on the equity if we sell, so we the heloc may not get paid in full after the sale of the home. I didn't sign the heloc. Am I legally responsible to pay 1/2 the outstanding debt? The heloc went for a car (i have) and downpayment on a property we own jointly with others (listed as 4 individuals) and a variety of other improvements. we registered as domestic partners in King County, but have no legal marriage agreement (washington state doesn't recognize gay unions)