The Dutch

[publicunofficial]

Aww. I miss the days when I got to "discover" things like this.

To answer your question, the cause is primarily religious. They're mostly Dutch Reformed types.

Even Washington has a Dutch enclave in Lynden. Here's the gay marriage map for Whatcom County, courtesy of Alcon. You can probably figure out where it is.

IIRC, Bellingham (The green city in the south) is one of the least religious cities in America, while Lynden (red area in the north) is one of the most.