Homosexuality is not the same as incest, or bestiality, or even polygamy, for several important reasons. First, animals (or children) are not capable of consenting to a relationship, so that's a completely different situation. Further, incest is usually exploitative in the same way, and biologically dangerous to any progeny for genetic reasons. And it is not an "orientation" that limits the possible marriage to that category like it does with gay people. So the comparisons are not appropriate.
Gay couples and families have existed from the beginning of time. The ONLY question is whether we continue to single them out for legal and financial discrimination. Notwithstanding the question of WHY we'd single them out for the denial of legal and financial protections, it is simply against the law to do so, since equal protection under the law is guaranteed by the state Constitution in California.
Marriage is, in this context, solely a legal right and set of protections given by the state. It has nothing whatsoever to do with religion. In fact, it is, and has always been, legal for a church to "marry" gay couples - they just don't get state benefits. It is likewise legal, and always has been, for a church to elect not to marry any two people. Churches get to do what they want, and rightfully so. Gay "church" marriage is already legal. So this really is ONLY about the state benefits.
It seems to me that the only motivation for denying legal benefits like inheritance, hospital visitation rights, and child custody protections to gay people is sheer cruelty, and maybe coupled with a curious impression that one's restrictive church doctrine ought to apply to non-adherents of that church - a bizarre and unsupportable position to hold.
Capslock