Before I get rolling on Part III, let me make it clear that I am not an attorney, and there is legal discussion to follow. This is my argument based on what I have read and how I apply it to this situation, and why I think it supports my argument. I know at least two attorneys read this blog, and I'm sure they'll correct me if I am way off base. :)
Because, IMO, no attorney would argue this as a First Amendment case (on either side, in my opinion), like the Loving case, the legal challenge would probably come from the Fourteenth Amendment Due Process and Equal Protection Clauses. (Federal applicability and challenges would be Fifth Amendment cases, IIRC).
In the Loving decision, Chief Justice Earl Warren wrote,
"[t]he freedom to marry has long been recognized as one of the vital personal rights essential to the orderly pursuit of happiness by free men."
At this point, numerous lower courts have held that the Loving precedent does not apply to same sex marriages, due to the historic context of the Loving case; however, with the passing of the Defense of Marriage Act (1996), I think the door is open for a Constitutional challenge of the legality of that act (via the Fifth Amendment) and its definition of marriage as well as CA's Prop 8 (via Fourteenth Amendment). As others have pointed out in comments, the historic context is beginning to mirror that of segregation and the Civil Rights movement.
If marriage truly is a "vital personal right" as Justice Warren wrote, then civil marriage should apply to all persons-- black, white, hetero- or homo-sexual. The Declaration of Independence stated that "all men are created equal," but Prop 8 (and Defense of Marriage Act) limits equality to those persons who are heterosexual.
(more to come...the more I write, the more I find to say)