Why do I want to get married, since I cannot legally grant that same public status to my other love? This is... a long and complicated situation, I suppose, but I'll give it a whirl to see how well my mind can spew random answers. (Most of my reasoning is not tangible; specifically the important parts. So I'll give the small reasons first and see if I can hit the real reasons on the forward swing.)
All of the standard legalistic reasons -- health, taxes, etc. While these aren't much of an issue (health is not so severe a problem for me as it is for Gwynyth; while I have been described as 'an interesting amalgamation of medical problems' by a friend, not even in combination are they anything much more than INCREDIBLY ANNOYING, aside from the much ranted-over smoke problem) I do admit that they are there not as a factor in the decision, but as a thing to be noted.
I imagine that I will wind up drafting some very strange collection of legal documents with respect to my status such as health care proxies and trust documents to make certain aspects of the emotional relationship legally present. Therefore, I merely note the legal statements as something which I am aware of, rather than anything that actually motivates me.
A minor thing to be noted is that I do intend to have children, though not at any point in the near future. Having a legally obvious guardianship without having to draft more documents (as a legal secretary, I'm beginning to have a certain wary fear of the amount of paper my eventual estate planning will generate, and I'd like to not have to decimate an entire forest to prepare it!) and the like having a clear other parent, even if such is not literally the case, is something which I desire.
I also happen to enjoy the prospect of making a public celebration of my love; it will be a small ceremony, but it will affirm for the world to know my intentions to stay with him for always. The existence of this public ceremony has a few useful side effects, such as keeping parents from grumbling (religious issues, as well as generalized "you're too young to...." issues).
I know that I cannot legally (both by law and in the eyes of the law) give that same sort of ceremony to my other love. However, we will be holding our own personal oathgiving at some point -- in order to make that feeling as 'official', where it matters; in our own lives and in the view of our respective deities.
I see no reason, in my own life, as always -- to deny that bond to one love because I cannot give it to another. It is something which matters to me in a way that I cannot usefully express; an inability to grant a certain sort of official recognition to someone else who deserves it does not, to me, mean that I should not give it at all.
To me, a marriage is not about letting the state into my relationship. On the contrary, it is making my relationship, my love, something of public respect, an undeniable bond. Perhaps a perversely twisted part of my mind is looking at it as a proof that I can and will do what I will, love who I will, within and outside of the mainstream and expected ceremonials. Part of it is the showing, the demonstration of my love, and a public affirmation of the oaths that I have made and reaffirmed many times in private.
I am not shy of my love, and I will not deny any of it if it is asked of me. I will, in fact, not only not deny my love, but bring it out and make it a part of the world around me. The ceremony itself does not change anything which matters to me.
The promises I have made are made, the oaths I have sworn unbroken. These would remain so whether or not I said "I do" before a justice of the peace or a clergyman of some religion or other. I consider myself wed to both my loves in my own eyes and the eyes of my gods. A marriage is, to me, not a time to make promises. It is a time to reaffirm those which I have made, publically, in such a way that they are Known. The fact that only one of these reaffirmations can be legally binding by the structure of law as of the moment that is irrelevant. The point is in making the affirmation in a way that cannot be denied.
So I will marry the man who will be my husband by law in a way that the law sees fit, in a way that the community will understand immediately. I will reaffirm that promise as well; having made it publically, I will still continue to make it privately, explicitly and implicitly, for so long as I can.
I will marry my other love in a way the law does not accept, perhaps, nor understand, which has no official bindings or meanings save those which are explicitly written up in individual legal documents. That will, however, hold the same meaning to those of us who swear, and to those who witness; and I will likewise reaffirm that promise every time I am able.
The law matters nothing, in truth, in such things; it is the oathtaking before witnesses, the restatement of promises that have been made and remade in private, that matters to me.
I have already made the promises I will remake; I merely wish to make them before the rest of the world, in one way or another, so that these promises cannot be doubted or denied.