Hillel Levin makes it, first with an important point about Jewish and secular law:
Judaism already treats Jewish and civil marriages differently, and synagogues—like all religious organizations—are free to define marriage according to their own religious principles. For example, marriages between Jews and members of other faiths are not performed or recognized in Orthodox synagogues. Other denominations perform them as they see fit. The same approach can easily be applied to same-sex marriages. Orthodox synagogues will not be forced to redefine religious marriage on account of the legalization of same sex marriage.
He connects this argument to the American tradition of religious pluralism and tolerance:
Unlike our Christian friends and neighbors, Jews grow up with our minority status deeply ingrained and without the instinctive expectation that our religious traditions and beliefs will naturally be reflected in the broader law and culture. As a minority within a minority, Orthodox Jews recognize that we reap the benefits of pluralism, tolerance, and accommodation. After all, if religious beliefs in this country were to orient secular law, we would find ourselves deeply disappointed and possibly threatened, just as we historically have in every other diaspora country. For good reason, then, American Jews and Orthodox Jews in particular are usually reticent about imposing our religious values and views on others.
(Photo of their traditional Jewish wedding by flickr users ed and eddie)