I'm really understanding your situation as it was really the only thing I was worried about after we decided to romantically pursue other people.
I'm in Eugene, and I'm finding a lot of people around me are very keen on what is going on in my wife and I's relationship as we are kind of immersing ourselves into the Eugene culture here and open about our relationship. Most realize that we're doing something right because it is easy to spot how real our happiness is.
I don't know about anywhere else in the state, but it seems like Eugene has a lot of people who are welcome to the mindset. There is even a few authors of Polyamory books published from here. Portland, from what I've sniffed around online, is supposed to have an active local polyforum like this one out there if you search for it.
Best of luck and stick around and keep us posted.
Media Specialist/Pop Culturalist/Eugenian/Native Louisianian/Professional Weirdo