Yes, this is civil disobedience. Civil disobedience by an elected official, which carries even more weight. I think the comparison to Martin Luther King, Jr. (who, whatever his failings, was not a brat ;)) is appropriate.
The only way to challenge an unconstitutional law is to break it. There must be a real case on which the court can base their decision. The Mayor (and the women who married) may suffer consequences, but they did what he believed to be the just and right thing.
I have always believed that forbidding gay people to marry is immoral and unjust, and today I am proud to live in a city whose public officials are willing to take a stand against injustice.
Besides, there’s just something beautiful about two women who have been together for 51 years finally being able to marry.