Precedent already holding that (i) marriage is a fundamental right, (ii) laws motivated by animus towards gays are unconstitutional, and (iii) choice in certain family-related matters and inter-personal intimacy is protected, thus combines with more-general holdings on freedom of belief and association to provide a substantial foundation for the Court's assessment of gay rights. The conclusion that denial of same-sex marriage violates equal protection or a fundamental right could thus be framed as only an incremental step.
Federal Power, Economic Liberty, and the Individual Health-Care Mandate
Even broad precedent on congressional power carries significant limits, contrary to an argument against the ACA. In National Federation of Independent Business v. Sebelius decision, a 5-4 majority found Congress's "commerce power" did not authorize the ACA's individual mandate, though another 5-4 majority (Roberts joining the liberals) held that provision to be authorized by the taxing power.
The commerce power ruling falls short of the full-constitution approach, yet was embraced by many conservatives because it portends future narrow holdings on federal power:
Conservative: Since forcing a purchase is not regulating commerce, the Court rightly held the individual mandate to be outside the commerce power.
Whole Constitutionalist: The Court slighted precedent that has interpreted federal power broadly, as territorial and population growth and revolutions in industry, transportation, and communication increased the integration of our national economy. For 75 years, the Court has held Congress to have power even over matters substantially affecting interstate commerce, including minor things that aggregated have such impact. That precedent covers the health insurance market, substantially impacted by individuals who would shirk coverage.
Conservative: But Congress has never been held to have power to compel doing something.
Whole Constitutionalist: That's not true or relevant. Congress mandated musket possession in 1792. The Court upheld requiring everyone to accept paper money in 1871. And it has never struck a military draft's service mandate. Further, no constitutional text limits the means Congress may use to exercise its powers, besides prohibitions on infringing individual rights, such as due process; no prior case precluded mandates; and mere infrequency of its past use of mandates cannot disable Congress today.
Conservative: But that would mean Congress's commerce power is unlimited--they could make us eat broccoli.
Whole Constitutionalist: No. Forced feeding violates unenumerated rights to bodily integrity. Most other assertions of federal power would still need to relate to matters substantially affecting interstate commerce and not infringe individual rights; and precedent precludes laws that would commandeer state officials to achieve federal ends.
But many conservatives despise the Court's broader judgment, upholding the individual mandate as authorized by another textual source--the Taxing Clause--and not finding any infringement of economic liberty.
Whole Constitutionalist: But the mandate was correctly upheld under taxing power. Because the 1788 Constitution's text allows Congress to tax for the "general welfare," it may use tax burdens, like the penalty for violating the mandate, to deter nationally-detrimental financial behavior that undermines the general welfare.
Conservative: I disagree. And regardless of the source of federal power, forcing a purchase still violates individual liberty.
Whole Constitutionalist: The constitution protects individual liberty from federal and state action to the same degree, and the ACA is no more invasive than many valid state-level economic mandates. Due process clauses limit both Congress and the states, and precedent upholds, against liberty claims, states requiring citizens to buy many things, such as food for children and safety features for homes. If economic liberty extended so far as to invalidate the ACA, it would also invalidate state building codes, child welfare laws, et cetera--which makes no sense.
The individual mandate's loudest opponents combined a general hatred of the policy itself with a too-narrow appreciation of our constitution; others argued federal power to be limited by economic liberty principles beyond what due process precedent teaches, an argument that also disregards our history.
The ACA, instead, respects the American concept of constitutional liberty, which emerged from two watershed transitions. First, in Reconstruction we responded to our depleted trust of individual states with basic rights of their inhabitants by adopting amendments that limited state governments--abolishing slavery (in the Thirteenth Amendment), providing several generally-framed rights protections (in the Fourteenth), and precluding racial discrimination in voting (in the Fifteenth). The Fourteenth Amendment's broad provisions now limit states to a degree very similar to the Bill of Rights' limits on federal action. For over a century, understanding of individual liberty thus has developed more in the frequent Fourteenth Amendment "due process" claims against state governments than in the fewer cases against the federal. But in general, any limits constitutional liberty places on states also restrict the federal government, both being subject to identically-worded due process clauses. Likewise, the long list of cases denying the much-more-numerous challenges against government action in various states has established a broad scope of things government--state or federal--may require without infringing liberty, a scope the ACA does not exceed.