If we believe, as the Constitution says, that each individual deserves equal protection of the laws, then marriage between any two individuals is constitutional. Nay, I say, I go further. I say any marriage of any number of individuals is protected. Government has no power to say that multiple marriages are not protected, so if a man can persuade several women to marry him it is protected. The same for woman who wishes to marry several men. The same a man who wishes to marry one or many other men or a woman who wishes to marry several women. I add to this list men or women who wish to marry a person of the same or opposite sex without any romantic or sexual intent. Why? There are legal protections and privileges invested in marriage recognized by the state. Therein lies the problem with marriage recognized by the state.
The solution I advocate is to pry loose the association of marriage and the state. Civil unions, invested with the contractual elements, without the religious, is the true province of the state. Civil union is a contract and should have invested in it all the rights, privileges, and the proper adjudication that all contracts are subjective to in the courts.
Citizens would not have to apply for a civil union; it would be a contract drawn up by their lawyers and registered.
Religious elements of marriage belong to the varied religious denominations and sects. One may have a male friend, if a man, who we wish to have the legal right to inheritance rights, decision rights in cases of medical emergencies, or the various other legal rights a chosen relative may have. It does not and should not have in it necessarily a romantic or sexual content. It is simply a contract between two or more consenting adults.
If we wish also to marry and can find a religious group to accept our beliefs of the number and gender of our bond, that is a separate consideration. Catholics, for example, would have to satisfy Catholic doctrine; whereas X religious denomination may accept same sex marriage or multiple marriage. It is not the concern of government their marriage, only the legal aspects constitute the concern of government.
Some individuals may wish to have a marriage without a civil union, thereby have a union with all the religious elements without the legal. Their legal rights and obligations remain those of individuals, yet have the loving and religious rights and obligations of their religious denomination.
We should not confuse and mix the legal and religious.