It's clear that, halachically, all contracts elevating same-sex relationships to the level of unions are proscribed as a matter of Noahide law. (See midrashim on Bereishis 6:2, and BT Sanhedrin....somewhere between 54b and 59a. I'm still looking)
It's nearly equally clear that there is no solid logical reason to prohibit said relationships and unions, and there even might be salient social policy arguments in favor of the practice. This means that the only remaining objections are axiomatically theological.
Still, if one takes one's belief in Torah MiSinai seriously, one finds oneself intellectually bound to believe in the axiomatic immorality of homosexual unions.
So does that mean that one is therefore obligated to directly support Prop 8?
I'll say no.
I just read an article in the Chovevei journal that illustrated the difference between protesting the objectionable and actually having to put a stop to it.
At first glance, one might think that the Mormons' effort provided the paradigm that all "right-thinking" people should follow, particularly Orthodox Jews. ("Da'as Torah" proponents are usually very uncomfortable when religious groups out-frum the frum in formulating public policy.)
However, I think it would be both morally and religiously sufficient to simply--abstain. This is a case where one could be yotzei "protesting" by simply not voting against Prop 8. (And be clear that, even if one's extrareligious conscience is bothered by ostensible religious "homophobia", the Torah's baseline position is not, to say the least, congruent with the current socio-political zeitgeist.)
Why would one not be obligated to go vote for Prop 8 given the opportunity?
There is an argument among liberal cognoscenti (and less-then-cognoscenti) that the full granting of sexual and marital civil rights is an inevitability. This is arguable in either direction; however, the fact that voters in thirty states voted to ban gay marriage does not necessarily postpone the inevitable. Aderabba: one can be certain that when Brown v Board was decided in 1954, even up to the passage of the Civil Rights and Voting Rights Act in 1965, the majority of (at least) the South's (white) citizens were dead set against the granting of ANY legal equality. One must remember, too, that the objections--when they resembled anything rational--were usually either Biblical (Genesis 9:25-27) and/or sexually Darwinian (even now, black male-white female love scenes in mainstream Hollywood are extremely rare). Yet the legislation passed, and has become an inexorable part of our political culture and zeitgeist.
One can claim that the Mormon effort might actually speed up the process. (One should also appreciate the irony of a Mormon effort to pass legislation restricting marriage to one man and one woman. In contradistinction to the "Da'as Torah" influenced fear of other religious groups' out-"frumming" Jews, I thank Hashem Shelo Asani Fundamentalist. They actually make us look good.)