In 1923, the Supreme Court in Meyer v. Nebraska noted:
While this Court has not attempted to define with exactness the liberty thus guaranteed, the term has received much consideration and some of the included things have been definitely stated. Without doubt, it denotes not merely freedom from bodily restraint but also the right of the individual to contract, to engage in any of the common occupations of life, to acquire useful knowledge, to marry, establish a home and bring up children, to worship God according to the dictates of his own conscience, and generally to enjoy those privileges long recognized at common law as essential to the orderly pursuit of happiness by free men.
The "right to marry" was thirty-five years later reaffirmed of fundamental importance in Loving v. Virginia, in which race was deemed an illegitimate barrier to entry:
Marriage is one of the "basic civil rights of man," fundamental to our very existence and survival.
Tradition that allowed such a barrier was not the final test as noted in Lawrence v. Texas:
[T]he fact that the governing majority in a State has traditionally viewed a particular practice as immoral is not a sufficient reason for upholding a law prohibiting the practice; neither history nor tradition could save a law prohibiting miscegenation from constitutional attack.
The same applies to various other changes in the marriage institution with constitutional implications, such as criminalization of spousal rape, once deemed a violation of marital privacy, particularly if the spouse testified.
Marriage is so much of a fundamental right that it could not be barred, so said the Supreme Court twenty years later in Turner v. Safley, even in prison:
First, inmate marriages, like others, are expressions of emotional support and public commitment. These elements are an important and significant aspect of the marital relationship. In addition, many religions recognize marriage as having spiritual significance; for some inmates and their spouses, therefore, the commitment of marriage may be an exercise of religious faith as well as an expression of personal dedication. Third, most inmates eventually will be released by parole or commutation, and therefore most inmate marriages are formed in the expectation that they ultimately will be fully consummated. Finally, marital status often is a precondition to the receipt of government benefits (e. g., Social Security benefits), property rights (e. g., tenancy by the entirety, inheritance rights), and other, less tangible benefits (e. g., legitimation of children born out of wedlock).
Thus, a same sex marriage in prison was discussed here. The importance of marriage traditionally is suggested by the first case -- a group of activities (such as establishing a home, intimate relations, raising a family) was held to require a certain institution, with public benefits and private responsibilities, to work. Thus, a ban on interracial marriage was problematic because if you wanted live together and so forth, religious marriage was not enough. To live out such a marriage, with its religious and other aspects, you needed a marriage license. This still holds true in the minds of many.
For these, the road is clear: if the government will provide, it must provide equally. The equal protection clause is general, not limited to race. It applies to sex and sexual orientation. Some accept marriage but try to explain that "same sex" is different. There are ways to do that just as there were ways to wall out interracial, inter-religious, poor, handicapped, or any number of other couples.* But, avoiding illegitimate discrimination is much harder.
This holds true if you think "marriage" is a bad idea. We can't dismiss the value of equality because we don't like the government benefit. Separate is not equal; "near" equality is not equality. So, much firm opposition to government involvement in marriage is fine and all, but somewhat besides the point. Some in my experience appear to miss this point, not impressed that another group wants to get "theirs" when marriage is problematic enough as it is. Some in the LGBT community thing this too. Again, that's nice and all, but there is this equal protection rule, and it applies even if you don't like the benefit.
And, many still support government involvement, but want to use the term "civil unions" or whatnot. This is so because a "public" commitment with state benefits tied to it continues to make a good deal of sense, even if the religious and private aspects can be done separately. [As it can be done now, Catholic churches not marrying certain people who are divorced etc., but said people are married civilly.] Or, if certain benefits are not just given to "married couples" but others as well, such as elderly siblings. This suggests there are various inequities; but, barriers to same sex couples still would be one of them.
Many things once only done via marriage, including cohabitation and even raising children, can now be done without marriage or civil unions. Thus, the constitutional right to use contraceptives was first secured for married couples, deemed part of marital freedom, but later was expanded to the unmarried too. Ditto sexual relations in general. But, we continue to have the government sponsored unions, now generally known as "marriage," because we continue as a whole to deem it a fundamental institution in society, one that requires some state involvement to work best. Call it "civil unions," the same basic principle would apply.
This was the case throughout history, and the traditional "right to marriage" continues to be deemed an important fundamental right protected by the U.S. Constitution, particularly as a "liberty" or one of those unenumerated rights secured by the 9th Amendment. But, putting that aside, as long as the state provides certain "privileges and immunities," as it does via marriage, it has to be done equally. Classifications can be made, but equality requires that they are reasonable. Race and so forth does not meet the test. So, even if there was no "right to marry," marriage could not be a benefit handed out in an inequitable way as it now is given same sex couples -- who are just as able to enjoy the aspects of marriage as those in prisons -- are deprived of their rights.
As protected by the federal Constitution, which is the supreme law of the land, even if democratically enacted laws violate it. The federal courts have a role in protecting such law in part by carrying out trials to secure the victims of such illegitimate laws. And, to protect serve this role, we have open and public trials:
openness to the proper functioning of a trial; it gave assurance that the proceedings were conducted fairly to all concerned, and it discouraged perjury, the misconduct of participants, and decisions based on secret bias or partiality
This is an important value, one that deserves to be honored broadly. [See last extended entry.] It would "undermine public confidence" and limit their knowledge and understanding if it is done in a overly modest way, particularly if the case is of high importance, be it a major criminal trial or one that touches upon the basic civil rights of man.
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* Discrimination by race, sex, sexual orientation, religion or poverty (in certain cases here) is illegitimate, so appeals to polygamy are somewhat besides the point as much as appeals to bestiality and many forms of incest (both which have consent problems; notably, however, quite a few states allow first cousin marriage). Interference with family dynamics also helps justify limits on incest, though when adopted or step-siblings are involved, this might get messy in certain situations.
Still, as Griswold v. Connecticut, the contraceptives case noted:
We deal with a right of privacy older than the Bill of Rights - older than our political parties, older than our school system. Marriage is a coming together for better or for worse, hopefully enduring, and intimate to the degree of being sacred. It is an association that promotes a way of life, not causes; a harmony in living, not political faiths; a bilateral loyalty, not commercial or social projects. Yet it is an association for as noble a purpose as any involved in our prior decisions.
Marriage is a "bilateral" loyalty under our tradition. It becomes much more complicated when more than two parties are involved. A 1:1 union is of different character, including pertaining to questions of decision-making and so forth, than a 1:>1 union. Polygamy can be imagined (see the dissent here), but for this reason plus problems tied to how it often is practiced here (consent issues, gender issues, in particular), it is not the same.
"Any" marriage isn't the test. A marriage without illegitimate barriers to entry based on discrimination is. This is what is being fought here.