Although many Christians base their opposition to same-sex marriage on verses like Leviticus 18:22 in the Hebrew Scriptures (and about 6 similar verses in the Hebrew and Christians scriptures), the overwhelming majority of American Jews, whose ancestors wrote and preserved the Hebrew Bible, support legalizing gay marriage.
A survey reported by the Public Religion Research Institute on 2013-APR-26 found that 81% of American Jews support legalizing gay marriage compared to 52% of all other Americans. 1
It is true that only 48% of Jewish Republicans support legalizing same sex marriage, But, with the single exception of LGBT Republicans themselves, Jewish Republicans were more favorable toward legalizing same sex marriage than other group of Republicans, and by a very wide margin. 2
Marriages between gay and lesbian Jews are increasingly accepted by Jewish religious groups. Reform Judaism officially announced its support for gay marriage during the 1990s. Conservative Judaism followed in 2006.
No Orthodox groups accept same sex marriage, though the issue of homosexuality has been the subject of some debate among the Modern Orthodox.
Polls conducted by the Pew Research Center in 2008-2009 measured American Jewish support for same sex marriage at 75%, and at 76% in 2010.
Jews may be more accepting of legalizing same sex marriages because Jewish law makes marriage a contract between two partners, rather than a Divine sacrament. This is why Jewish law has always permitted divorce if the relationship fails.
This is also why for almost all Jews, a civil marriage of two Jews performed by a judge, is preferable to a religious marriage of a Jewish-Christian couple in a church. In the case of a Jewish-Christian couple, my experience over 40 years is that most Christians would prefer a religious wedding, even a Jewish wedding ceremony, to a civil one; while most Jews would prefer a civil ceremony to a church ceremony. The reason why almost all Jews would prefer a civil ceremony to a Christian one is because Jews see Christianity as undermining the identity and solidarity of the Jewish people. Secularism -- while also a threat -- can be lived with, especially by Jews who are not that religious themselves.
This also reflects the Jewish emphasis on belonging as more important than believing. An agnostic Jew can join any synagogue in the U. S. A Jew who believed in Jesus as the Messiah and as the son of God would not be welcome anywhere. Thus, different religious concepts about marriage in general, make Jews more open to same sex marriages in particular.