I’ll give you 1a for free. 
Thanks for the lagniappe 
Now onto #1b. I don’t know what you’re referring to when you say “financial and biological need.” (I agree that if a woman is pregnant and not in a position to go on maternity leave, she’ll most likely be supported by her husband until she’s able to go back to work. Is that the “financial need” you’re talking about, for the man to be the breadwinner? If so, that is only be true for 9 months.) Other than that, I don’t see any other financial/biological need for the woman to be the homemaker, and the man to be the breadwinner.
Yes, that’s what I mean; the financial/biological pairing required for spouses to produce a child. Prior to WWII, women were actively discouraged from competing in the workplace, and let’s face it, men are biologically exempt from being pregnant. The presumption was (and still remains) that men and women marry to create a secure nest for raising children. By agreement, a woman’s work was to keep house, and a man’s was to pay for it, so it makes sense that legally the woman was at greater risk of getting left out in the cold due to being financially dependent on her working partner. Marital law reflects the historical legal precedents created by financial/biological pairing.
As for #2.. The premise here is that same-sex couples are double income, childless families. But same-sex couples may be, and often are, single income, +children families. This may be for a variety of reasons: they have adopted children, they have other children from previous relationships, or one (or both) of them have been artifically inseminated. So the premise that same-sex couples are DINKs does not hold. (But the most important point here is that the needs of one family over another should not entitle them to any extra legal benefits. Family needs should to be taken care of by the family.)
Yes, times have changed but the greater percentage of marital pairings today still follow the historical model, so existing marriage and divorce entitlements remain justified in that context, i.e. the context of providing financial security for those (and their children) who by agreement, rely on a breadwinner to raise a family. Civil unions accommodate the legal needs of other marital pairings resulting from today’s economy and biological alternatives to personally becoming pregnant, thus avoiding the financial necessity of a relying on a breadwinner. So, marital law is necessarily different than civil union law, verbal agreement, etc, and therefore justifies different legal benefits.
Please note that I’m not claiming that current law, as it applies to marriage, civil unions, etc, is perfectly just; it’s a work in progress, and progress towards better law that reflects today’s realities needs to be made. And there’s the tax issue, which no one believes is just to begin with… however, if married couples get better restitution from an unjust tax code, are we not to some degree claiming sour grapes? My main point is, the legal reality of a breadwinner and his pregnant spouse is necessarily different than that of two breadwinners living together with an adopted child, so claiming that all families ought to be constrained by the same contract doesn’t reflect the reality of diverse family situations.
Also, FMS = free market society.
Ah, that makes sense… thanks for the clarification.