Does anything in Jewish tradition forbid a rabbi from officiating at non Jewish marriages? Not really. Until now, no one would have asked a rabbi to do such a thing. As a rabbi, I would make some changes in the traditional Jewish ceremony so that it would apply to the situation at hand. But my power as clergy in the US still makes it a legal wedding under US law. It may not be a valid wedding under Jewish law, halachah, but since these are non-Jews, whether or not it is valid according to Jewish law does not pertain. Many of the symbols of a Jewish wedding translate beautifully in to any wedding – a marriage contract where the two parties spell out their commitment to each other, a wedding canopy signifying the new home being created, a series of blessings for the new couple, and a broken glass to remind us of their commitment to each other in good times and bad. It is a mitzvah to share a Smicha with friends, and any certified marriage performer should be delighted to extend friendship along with civil authority. and these blessings come back to the whole community. The Orthodox rabbi on staff at UM Hillel officiated at the wedding of the maintenance man (not Jewish) to a non-Jewish woman.