"Those who are clergy who are supportive of the right of gay couples to marry really have only one option open to them. They should refuse to do any further legal marriages. No-one can force anyone to solemnise any marriage – a fact which has been much glossed over by the media. Some couples in some circumstances have the right to marry in particular places but not the right to be married by any one person." (Father Kelvin Holdsworth)
Read more agitation at WHAT'S IN KELVIN'S HEAD?
Actually, I do know a Unitarian minister (she’s retired now) who did this.
This idea has been floated in the Episcopal Church in states that allow neither same-sex marriages or civil unions.