I want my boyfriend of 4 years to adopt my daughter. Do we have to be married?

My daughters biological father is somewhere across the country and we haven't spoken to him in over 6 years. My boyfriend and I have been together for 4 years and he has become "dad" to my daughter. They have a great relationship and he considers her his own. We want to have him adopt her but don't plan on getting married anytime soon. We feel this would be a good idea especially if anything were to happen to me. God forbid she have to live with deadbeat by default. Do we have to be married for this to happen or can we go through the courts if the real dad allows this. We live in WA state.