There are some nice spas and saunas in Washington and Oregon, and many of the hotels in California Wine Country cater to adults and, at least in my limited experience, are thong-friendly. I would expect hotels in Oregon Wine Country to have a similar attitude.
I don’t do much lap swimming myself, but other members of the community have worn thongs at some of the pools in Washington without any issues. And most counties in the state do not prohibit thongs, so you can wear them at the waterfront parks.
When I look for places to stay, I try to find hotels that only or primarily have double-occupancy rooms. Even if they aren’t adults-only, there tend to be far fewer children than you’d find at places with rooms that accommodate families.