My views in relation to your suggested questions in relation to applications of either the Fifth or Fourteenth amendments.
Now let's look at the issues that must be answered in a Fifth or Fourteenth Amendment challenge to gender restriction. First, is the restriction unsupportable? Second, does it apply to everyone? If both are true, it violates the due process clause.
Whether the restriction is unsupportable is a matter of assessment and I fear that on any factual assessment, the answer will be no. Historically, same-sex marriage has generally not been legal. That the vast majority of the States in the US, and countries in the World still maintain this restriction.
Arguing the opposing view, recent moves by any number of countries to legalise same-sex marriage, or civil unions or partnerships may be indicative of shifting attitudes and norms, and whilst it was not unsupportable before, it will likely be very shortly and the law should reflect this imminent change.
Does it apply to everyone? The answer here, has to be, in my humble opinion, no. It only applies to gays and lesbians. Whereas the restriction against marriage based upon race could be argued to have impacted the entire population - any white man would be prohibited from marrying any black woman of his choice, or any white woman would be prohibited from marrying a black man of her choice. In the case of same-sex marriage, I think an argument along similar lines won't carry much weight. If the Court takes a narrow construction - they can limit it to gays and lesbians a broader construction may apply it to bisexual people and transgender people - therefore encompassing the GLBT community - but any argument beyond this is reaching too far, I think. No court could factually accept that the same-sex marriage restriction would affect a straight man or woman.
Further, on my reading of the Fifth Amendment, this extract is most applicable (the balance dealing with criminal offenses, privilege against self-incrimination and deprivation of property) - "...nor be deprived of life, liberty, or property, without due process of law..." This poses a problem, I think. Yes on Prop 8 could reasonably argue that due process was afforded - it was put to a vote of the eligible population in California and the voters passed the Proposition. Is there a clearer affording of due process than this?
Then we move to the equal protection issues. First, is there a discrete class that is affected by the discrimination? Second, is this discrimination invidious? If both are true, the discrimination violates equal protection.
An argument based upon the Fourteenth Amendment, in my view, would present a far stronger case. There is a clear and discrete class affected by the discrimination - namely, the GLBT community. Is the discrimination invidious - unfair, unjust or likely to incur resent or anger in others? The answer is arguably yes. The evidence of resentment and anger is clear - factually, the Court cannot fail to take judicial notice of the reaction of the GLBT community to Proposition 8.
The relevant words of section 1 of the Fourteenth Amendment - "No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws." - are a little troubling for the following reasons:
1. The first part clearly works in favour of overturning Prop 8. It clearly abridges a privilege afforded to heterosexual citizens.
2. The second part emphasises the need for due process again - and as with my earlier point - I believe this may work for Yes on Prop 8 in an argument that due process has been afforded by public vote.
3. The last part can go either way, I think. A State's laws are what is in its law books. California laws govern marriage. Proposition 8 limits marriage to heterosexual couples only. Proposition 8, having been passed, is also law. Arguments for overturning Prop 8 could argue that the marriage laws afford rights to heterosexual Californians and the GLBT community must be afforded protection under that law. Yes on Prop 8 could argue that Prop 8 is itself a law, and
they need to be afforded its protection lest same-sex marriages bring about the apocolypse...etc.etc. (read with necessary sarcastic inflection).
I hope I haven't made too much of a fool of myself in outlining what I think the position may be... I am happy to be correct if I have misinterpreted anything.