Unitarian Universalists were lead plaintiffs in the Massachusetts case that legalized same-sex marriages there. The denomination's general assemblies have come out in favor of marriage equality, and our ministers do lobbying for the issue around the country.
The denomination's president, Rev. William Sinkford, is on the Human Rights Campaign's Religion Council. His essay on "Whom God Has Joined Together" is available on the HRC's website as part of the week celebration.
Still, I didn't know there was a week for marriage equality. I bet the cards are better than most Hallmark Valentine's!