A guy takes a camping trip up north and the thread grows in leaps and bounds. I’m going to try and reign this back in but there is a lot to cover.
Long post alert…
At core marriage is just another relationship between people although it is a very specific type of association. People associate and build relationships all the time, as they should since it is in our rational self-interest to do so and that is why Freedom of Association is a core political principle.
Certain relationships build due to personal interest between people and when it becomes deep and emotionally pleasing it can be a loving relationship. Note that this is still the broad sense of the term as love can exist as a parent for a child, two friends who share a common interest (think Francisco and Rearden), and this includes romantic love. Romantic love inters the field when this generalized category of love builds between two people and advances to a strong psychological/emotional/sexual state of valuing each other. It’s the difference between being really happy to see someone and fire works going off when you look at them.
Romatic love is the last state of the chain and in all honesty doesn’t need any other qualifiers or rules to dictate it outside of those dictated by the nature of man (i.e. value based, selfish, honest, etc.) Marriage is one institution that has developed to formalize this in a social context, not only serving as a simple identifier but more importantly it also serves a rational legal purpose in a complex economy by offering identification for business reasons (i.e. insurance) or delineating property rights when the couple does not do this themselves (chain of custody in an accident). It’s be a wide road to do this much like many things in society that were originally a product of pre-enlightenment society but that is the essentials as they boil down in an individual based society. We can see this roughly when we look at how marriage is handled today in Westernized countries versus the rest of the world.
Marriage is not necessary as a commitment but it is in your self interest as long as the laws support it (think of the old “marriage penalty” on taxes).
But more importantly, at least for me, the most selfish thing is that it is also an expression of love.
Sure, it is a construct born of tradition and serves a social context but it is also a powerful symbol of love from art as well. In affect, you get married because you want to slay the dragon and earn the hand of the fairest lady in the land, not because the tribe/God(s)/your neighbor says you should for someone else’s good. Marriage can and should (if you let it) represent a romantic love announced loud and hard and in no uncertain terms. This would be no different than opening your business out of passion and love for the work despite the name being nothing more than a legal construct in a social context. It is there for rational reason but it is not the purpose of you opening the business - The purpose is you, your life, and all the values that lead to the passion to act.
Now, as for what defines marriage, are its essentials.
1.Romantic Love. I think there is no argument about this. Classic definitions that remove this important and most critical aspect are to be laughed at for the archaic institutions that they are, just as we would any other institution mystics and skeptics have been allowed to abuse (Government, art, relationships, ethics, etc.). Marriage is romantic love, courtship (the chase), and most importantly it’s the part where you “live happily ever after”.
2.It is a relationship, therefore it follows it needs to be between two people.
3.The two people are planning for their relationship to last a long time and have interest in making it an open commitment to identify that fact for themselves (a primary) and others (secondary). Primarily it’s just to tell each other that you love each other then plan from there, see point one above. Secondarily, if someone asks my girlfriend out, he can be excused for possibly not knowing that she was in a long term committed relationship. Here the gentlemen’s agreement, first time is a compliment, is in order. If I marry my girlfriend he KNOWS that she is in a long term relationship and needs to act accordingly. Someone will take that out of context I’m sure.
4.The two people involved are planning long range and are identifying their relationship within the legal framework since it allows for property in their relationship to work smoother (by property I mean the classic definition to freely make choices in one’s life) and facilitate ownership and contracts.
As for the assertions as to what is also essential, I’ll refute those now:
1.Children. The ability to have children has no bearing on whether my wife or I decide to have a longer term romantic relationship. A potential is not a duty or an actual result, and I can say this for certain as I have been with my wife for 23 years and we have no children. Children are considered essential as a result of religious reasons (i.e. mystical nonsense with no casual link to reality) or the traditional/tribe mentality (a duty to propagate the species or support the community). I hope the obvious violation of basic egoism implicit in the later reason needs no elaboration. Also, if I were to accept the necessity of children to make a romantic relationship “more real” or to allow it to evolve to a final state of happiness, I would now have to go home and tell my wife that we are not really married since she is not pumping out babies like a broodmare. I’m getting to old to dodge flying objects so that is not happening.
2.Physical appearance. While I would allow for the idea that a romantic relationship likely functions different in some aspects based on whether it is man/women or man/man, that is like everything else, something identified and agreed to by the two people freely associating with each other and is more or less just a nuance of the nature of romantic relationships as a whole since each person is always unique to a relationship. But these subtle differences are personal and like all chosen relationships handled personally, not an essential characteristic of the definition itself. There is no essential detail in what amounts to the random organs of those in a romantic relationship that defines it or marriage itself. If my cousin marries a women or a guy, it does not change how the law deals with them or I, only how they deal with each other. Sexual organs are unessential of the points I listed above thus as a measurement it is omitted.
3.Marriage is an institution. So was slavery. Not to drop that cliché on the conversation but really, an argument from tradition is not an argument but a demand to do something based on nothing more than that is the way we have always done it. If I were to transfer this argument to an economic example I’d be shanghaied by the good folk on this forum, rightfully so, and the logic applies here. Marriage is like anything, it servers the rational self-interest of the individual. The sexual organs of those getting married are very important to each other, but of no consequence to others. I could care less what they are on my neighbor.
4.I’m force others to accept my definition. As tempting it would be to say, “Good, I’m being rational and they are not!” I’ll avoid that since it is bad form 
Besides, forcing people to think is a contradiction. The truth is they are being a touch irrational however if they think that someone getting married hurts them or forces them to do anything. It in fact forces nothing on them since they can still do what they please. They can have heterosexual marriages and condone them all day long. They simply cannot use political force to mandate others follow their belief, which is the only real force being applied. Side example, Christians demand that December 25th be about the birth of Christ and I ignore their definition since I care less what they think. It’s interesting and telling that I don’t care what they do on that day but they care what I do. This is the same thing in principle. “Traditionalists” are not forced to believe anything. They however do support laws that do force others to do something.