The source appears to be http://blog.okcupid.com/index.php/gay-sex-vs-straight-sex/
Here's a somewhat lame explanation:
"That right there is a map of who’s gay curious in the U.S. and Canada, where red is more instances of curiosity and blue is fewer. It’s from OKCupid, which runs a pretty excellent blog where they crunch the numbers they’ve amassed in their databases from their users and present their findings in an informative and witty way. Ironically, though perhaps not unexpectedly, Mormon Central Utah is just as orange as the rest of the West with a particularly red spot right where Salt Lake City is. Their explanation of the map:Using the incredible power of computers, we were able to break down this question geographically. Here are straight people who either have had or would like to have a same-sex experience in the continental U.S. and lower Canada. You can see some sharp geographic divides.
Awesomely, the mountain West lives up to its Brokeback reputation, and Canada is orange nearly coast-to-coast. Even in the yellow and blue areas, you can see pockets of gay curiosity in interesting places: Austin, Madison, Asheville. Anywhere soy milk is served, basically."