gavra, wrong. We can’t give any legitimacy to homosexual relationships, whether it’s civil unions or marriage. Homosexual relations are forbidden, and that’s that.
An interesting side point: If any recognition of the legitimacy of same-sex unions is wrong because they are forbidden (even for non-Jews), then I find it very odd that the gedolim have chosen to actively align themselves with the Catholic Church in this fight. Doing so gives the Catholic Church a sense of legitimacy* which is just as forbidden.
The Wolf
* Working with them does grant them legitimacy. The gedolim have long forbidden Orthodox Jewish organizations from partnering with Conservative/Reform Jewish organizations in otherwise-worth endeavors because doing so would lend legitimacy to them.