As part of my duties as High Priest of the Wicca, I've performed wedding ceremonies. My legal authority to do this derives from the Universal Life Church, which makes those marriages "as legal as church on Sunday."
The most recent ceremony I performed (in 2013) was for a same-sex couple. Just as it was legal for me to be an officiant, it was also legal under the laws of the state of New York for both their names to appear on a marriage license.
I would perform that service again in a heartbeat. I would perform it for any people, for any type of relationship, as long as all involved were consenting and willing to share their lives with each other. I will do 10,000 times if that's what it takes. If it ceases to be legal, I will do it 100,000 times, as long as the people involved accept me as their officiant.