There are a lot of liberals in WA, but even here, its not as liberal as you might believe. This state does have a DOM law (Oregon does not, and between the two, I think WA is more liberal), and legalizing gay marriage was NOT on the agenda of the last session of the Legislature. There was debate over putting sexual orientation in the state's anti-discrimination laws, and it failed.
On the other hand, there was a lot of outrage in the Seattle media over the anti-gay-marriage rally, and some people even blamed Safeco insurance company (the Mariners ballfield is named Safeco Field, for you non-sportsfans), cancelling insurance policies. The Mariners' PR people said they would make the field available to the other side of the debate, and my prediction is, if they did so, they'd fill the place. The Mayday for Marriage people did not have a lot of public notice of the meeting, no doubt the attendees were solicited by word of mouth, or through church announcements, so a bigger opposition could not be raised. If the event were publicized on Seattle TV, you might have had a few more attendees, but you would have had a LOT more protesters.