A practicing Jew and a non-Jew being married would not be "interfaith" to a Rabbi. It would be heretical, apostate.
As far as I know for Protestant Christians (I'm a better expert there), one does not need to return to their Pastor and get an annulment or divorce decree. It would simply be processed through the State.
So the idea that religion is irrelevant doesn't apply here because the religious basis I'm referring to is Scripture, both in the New and Old Testaments, referring to sexual behavior.
In this case, would the Gentile be a non-Jew by birth or a non-practicing Jew?
The reason I ask is that if one of the people being married says they don't believe in the the Jewish God or practice Judaism, then, yes, that would be an acceptable reason for a Rabbi to not marry them.
However, if a professing Christian gay couple were refused marriage in a situation where gay marriage is legal, then that refusal would be unlawful because both of the people being married profess belief in the God of that church.
There is societal derivation of marriage, which Western countries adopted and promulgated to promote families, etc.
I can't presume to know how the LGBT community in CA felt this week; but I do know that there's greater oppotunity for all groups, blacks included, to come to a greater understanding of the need to expand civil rights--absent marriage--of the LGBT community.
With respect to marriage, I can only comment on what I have seen. And plenty of us have been exposed to successful and unsuccessful hetero marriages. Don't twist my words about the qualitative nature of gay marriages.
In response to your final point, should no one be entitled to a belief system (because belief systems set up boundaries)?
I answer this below, but if gay marriage is recognized by the State, then there is no way that a church or other organization could refuse to marry a gay couple without it being construed as a political statement, thus setting up that organization for losing its tax-exempt status.
Their would be no flavor to society anymore because all societies, organizations and churches would be forced to eventually concede on the gay marriage issue, instead of offering a difference of opinion on the subject as they do now.
A rebuke like Prop. 8 represents an opportunity for all Caifornians to engage in a new dialogue which, first, does not involve the courts, and two, does not involve a concept like marriage which, in a modern sense, has a religious derivation.
EXAMPLE: At some point, a gay couple somewhere would demand that their church marry them. If the church were to refuse, it could be sued and lose its tax-exempt status. Therefore, no church could continue to be Bible-based, but instead would have to abide by what the State tells it is acceptable.
And no, I don't believe I'm saying "F you." I'm saying, "Don't go there."