This varies widely from movement to movement.
The vast majority of Reform and Reconstructionist rabbis recognize same-sex unions as sacred under their interpretation of Jewish practice and values. Conservative rabbis have a range of responses to this issue. Most recognize same sex marriage as “righteous” (and as of 2006, Conservative rabbis can perform them), but not “the height of holiness”.
Most Orthodox communities do not recognize gay marriages as legitimate under Jewish law. There are a number of Orthodox gay couples who choose civil marriage, or who live together as if they are civilly married. These men and women often prefer the orthodox way of Jewish living—they keep kosher, celebrate holidays in strictly orthodox ways, attend Orthodox synagogues either openly (where accepted) or closeted (where not). But in general these men and women practice orthodox Judaism on their own or with like-minded friends and family, outside of traditional Orthodox organized communities.