Marriage in the United States is an unnecessarily intertwined combination of two concepts. First there is the aspect of vows/religion. A couple makes promises to each other, often concerning loving, cherishing, respecting and remaining faithful to each other. These vows are made before a couple’s choice of witness, a minister, priest, judge, etc. Many churches have conditions attached to this service specific to that religion or denomination. The couple can choose who performs their service, which vows to express and the format of the service and subsequent reverie. This is ‘traditional’ marriage.
The other component is the legal one. States issue licenses to qualified applicants based on a very minimum criteria established by that state. Once the license is received, all that is required is signatures of the couple and the official, religious or civil that performed the actual service. The license itself does not contain a copy of the vows, a list of religious doctrines or any guarantees or promises of fidelity, love or sexual behavior/expectations.
Once the signed license is filed with the clerk, the marriage is official. With this the couple is granted many benefits and rights, primarily for tax filing, joint property, and the right for one spouse to be able to speak or act on behalf of the other.
Without the first component the second is really little more than a partnership agreement. The state does not concern itself with infidelity, family finances, love, child rearing or domestic behaviors and expectations unless those behaviors violate some other law, or upon dissolution of the contract/marriage.
By all appearances, the law, the state, sees marriage as nothing more than a limited partnership, a civil contract between consenting parties.
For these reasons when talking about ‘legalizing’ an aspect of marriage, we are limiting the discussion to the legal component of marriage only.
With any other type of partnership contract filed with the state, there is no requirement that there be only one male and one female unrelated adult as the only possible combination of partners in that contract, simply because there is no reason to. The state has no interest in the gender, race, family history, or sexual preferences of partners in a contractual partnership.
The only reason polygamy is excluded from the legal definition of marriage is religious influence/tradition.
If a law exists for no other reason than a religious one, it should be considered unconstitutional. Not all religions ban polygamy. Therefore laws created to ban polygamy are specific to one or more religions, and not to all equally, thus establishing one religion’s laws as superior to others.
There is no sound secular reason to prohibit a couple, or larger group from forming a contractual partnership under civil law. Regardless of the makeup, the civil component has nothing to do with sex, sleeping arrangements, parenting privileges, financial guarantees or division of domestic obligations and duties.
Consenting adults should therefore be able to form such contracts without regard to gender, race, religious creed, or sexual orientation.
As for the other component, the religious/vow component, those are not legally binding and are completely external to the discussion of legalization. If a private institution such as a church chooses not to honor a polygamous marriage, that is its right.