For the purposes of taxation and spousal benefits (et cetera), the government of [insert nation-state here] shall define marriage as the civil union of two non-closely related, consenting, adult, living persons.
Most of this blog post really isn't about marriage. It's about defining the words that define our definitions. Okay, let's parse the following in this order:
5. Non-closely related
7. Civil union
Who is a person? A modern human being is a person. An archaic human being, like a Neanderthal, was probably a person. A space alien who purposely traveled to Earth might be a person. An angel might be a person. A demon might be a person. A robot with human-level intelligence (or higher) might be a person. A god might be a person.
Sapience should define personhood, generally speaking. Sapience, not just sentience. Sapience is a cut above sentience. Sentience basically means consciousness and awareness, like any animal with a nervous system. I must have read this on Wikipedia, but in the Harry Potter universe, centaurs are a sapient race. Instead of being counted as persons, a category of legal status that includes humans and goblins and whatnot, the centaur race chose to be counted as beasts, another legal status that includes non-speaking magical animals and regular animals.
If a being is sapient enough to choose to be counted as a person, or choose to not be counted as a person, then that being is obviously a person.
What is being alive? Obviously, living things are alive. Biologically alive things are alive, even if unconscious or comatose. Is there a consensus whether self-replicating viruses are alive? Undead things might be alive. Vampires might be alive; they might be living persons. Zombies might be alive, but they probably cannot declare personhood for themselves, depending on what kind of zombie they are. Ghosts and spirits that can communicate with the general world of the living might be considered living persons. Sapient artificial intelligence might be alive.
If a thing seems alive, then it probably is alive (no guarantees).
Who is an adult? Adulthood is a number, an age, a line, that is determined by the society in which a person lives. Like life and personhood (and every other term we're defining), adulthood has a fluid definition. Is biological maturity considered adulthood? Is emotional maturity considered adulthood? 18? 21? 25? 500?
If a person commits a crime and is tried as an adult, then that person might be an adult but might not be an adult. It's complicated.
What is consent? Yes is consent. However, a coerced yes is not consent. Does a subtly manipulated yes count as consent? How about a yes brought about by duty and cultural norms? How about a yes caused by propaganda? How about a yes caused by pop culture? Do free will and moral agency even exist?
If both (or all) parties have a similar belief in free will, or the lack thereof, then consent is possible. Possibly.
How can we determine close relations? Members of an immediate family are close relations: Parent/child and sibling/sibling. Direct line of ancestry and descent are close relations, plus or minus several generations. First cousins are close relations, depending on the culture. Anything beyond that is non-closely related, hopefully. Keep in mind that a person has an exponential amount of ancestors (two biological parents, four grands, eight greats, etc., with redundancies), and that person has the potential to have a multitude of direct descendants (children, grands, great-grands, etc.). Time travelers beware.
If both parties are neither siblings, parent/child, grandparent/grandchild, great-grandparent/great-grandchild, or first cousins within (let's say) four generations, then they're not closely-related. Your mileage may vary.
What is two? Two is one plus one. Conjoined twins are two people, as defined by having separate brains, separate nervous systems, and having an overall separate sense of self. (Conjoined twins are also closely related, which goes without saying.) A person with a multiple-personality mental illness is one person, with one brain, and one consciousness -- and arguably, one personality at a time. A person possessed by a legion of demons, on the other hand, might be one or many, but that person does not have consent, moral agency, or control over his/her body. The legion of demons can probably be separated into individual evil spirits. The monotheistic Christian triune God is another matter altogether.
If a person has his/her own sense of self, due to having one's own brain (or soul/spirit/essence/operating system, for the disembodied), then that person is one. Double that amount of one, and the number becomes two.
Hopefully you noticed that I tried to avoid any language that refers to gender. In order to be timeless for the next ten thousand years, we must accommodate all persons -- humans, aliens, angels, demons, gods, monsters, robots, etc. -- regardless of gender, or lack thereof (e.g., angels and robots).
What is a civil union? This is where I stop most of my hyperbole and try to write something actually relevant to the world today. A civil union isn't holy matrimony, and the government stuff involved should be kept separate from any religious stuff. This specifically applies to the United States of America, and hopefully other present-day countries, as well as societies in the future, with religious freedoms.
I've read various people's opinions, and a common complaint is that they don't want the word marriage to be used when referring to the union of same-sex couples. Beside the context of their own denominational theology, these commenters seem to be hung up by the modern English word marriage. Specific words in specific languages are relatively temporary bits of communication. In the next ten thousand years, the word marriage might not even be spelt or pronounced that way at all. In any case, since state and federal governments involve themselves with joint tax returns and spousal benefits, a simple catch-all term like marriage would be pretty efficient (as far as governments and bureaucracies are concerned). A term like civil union would be equally efficient, as long as it is applied to all unions of two non-closely related, consenting, adult, living persons.
A second common complaint, which has happened in the real world, involves businesses that are open to the public, but these businesses to not want to be vendors at same-sex weddings --
Perhaps a reasonable compromise can be reached. A business open to the public must serve the public. A business that requires paid memberships might be able to be more picky about their customers, to a certain extent. A business that is wholly owned by a church/religious organization should be able to draw the line against same-sex weddings (and unions between humans and angels/demons/vampires/robots/talking horses). Then again, my proposed solution is probably full of loopholes and complications.
The TL;DR (too long; didn't read) message of this blog post is basically the same for all my recent posts: Be kind, and if you can't be kind, be clinical (i.e., how does this issue honestly affect your life?).
Photo from my Instagram profile: Yes, my middle name is Jeffrey.