I know the whole world (or at least the three of you who actually read my ramblings) has been holding their breath, waiting for me to weigh in on this subject, so here you go…
There are two primary aspects of marriage which need to be addressed separately; religious and secular. I will start with religion.
Every religion has its own specific guidelines regarding marriage. For the most part, the religious position is that some deity communed with one or more individuals and provided them with instruction as to how things are done. These instructions are frequently canonized in written form. In the US, roughly 80% of the population is Christian, the next two (marginally) significant religious populations are Jewish, and Islam (How on earth did no prayer in schools get passed? Sorry, off-topic).
I do not have access to a copy of the Torah, so I can’t speak to its entire content. I know a significant portion of it is the same content found in the Old Testament of the Christian Bible.
I have a copy of the Qur’an, and have read some portions, but not all. I claim no expertise and am happy to hear from those more knowledgeable in the text and its content.
Of course, the scripture most people drag out with respect to homosexuality is the Sodom and Gomorrah story (Genesis 19). This isn’t really enough to make the case though. Sure, it sounds like they wanted to do inappropriate things with Lot’s guests, but there are a number of things other than specifically homosexuality that could be the “Sin”. It isn't clearly spelled out.
The Qur’an refers to the same event, but it’s wording is rather more precise (S. XXVII. 50-58). The interesting excerpt is here.
55. Would ye really approach men in your lusts rather than Women? Nay, ye are A people (grossly) ignorant!
58. And we rained down on them a shower (of brimstone); And evil was the shower On those who were admonished (But heeded not)!
(S. XXVII. 50-58).
(This is taken from the Translation by Abdulla Yusuf Ali. I know there are other translations with slightly different wording. However the meaning is still very much intact in the others I have seen)
So, if you are a follower of Islam, it would seem it is fairly clearly spelled out, whereas the Christians and Jews may have a little room for interpretation. On the other hand, Genesis 2:24 states
24. Therefore shall a man leave his father and mother and shall cleave unto his wife; and they shall be one flesh.
That seems fairly clear. Man. Wife. Cleave. One flesh. Not sure how else you could interpret that.
Of course, if your faith believes in continued revelation, then you still have room for change. If your religious views allow for modern prophets to receive revelation from God, then you have the ability to clarify or change position, based on modern direction from your respective Deity. As I understand it most Christian churches do not believe in modern revelation( I am aware of three that do believe in modern prophets: Jehovah's Witness, Seventh Day Adventist, and The Church of Jesus Christ of Latter Day Saints. I imagine there are a few others I am not acquainted with), so apart from playing it fast and loose with the interpretation thereof, they are stuck. Or insincere.
But ultimately the religious aspect of marriage is of minimal concern. If you don’t like a specific belief system, either you conform to it, because you accept that it is the word and will of a higher power, or you find a different belief system that suits you (or you could just join a social club and skip religion altogether, since perhaps what you really want is a social system that fits you, rather than a deity to fit yourself to, but I am sliding toward off-topic again).
The more important issue is that of secular marriage. It is what is recognized by the earthly powers that be. It is what defines marriage for a culture. Religions may establish the moral bounds of the faithful, but it is the state that ultimately has the final say regarding temporal validity. Every religion ultimately conforms to the standard set by the state. You might get married in a church, but you still get a marriage license from the state.
So what is the state’s interest in marriage? Why does it even care who you are "gettin’ busy" with?
Since the state is “us”, you could make the case that our religious morals dictate what the state wants. That is likely partially true, though there are some limits in the US due to the 1st amendment.
But while religious morals of the masses explains some of the laws regarding marital behavior, it doesn’t explain all of it. It certainly doesn’t explain tax breaks and other perks for marriage and child rearing (Which is what we are really interested in, aren’t we? Otherwise, you could claim marriage by deity of choice, and live together sans license from state).
Consider the state as an organism. Whereas we are made up of cells and bacteria and the like, the state is made up of us. The state, like any other organism wants to survive. It wants to thrive. It must therefore grow. It needs more of us; more of us to produce stuff, more of us to think up better ways to produce stuff, more of us to think up new stuff to produce.
The state wants kids (I assume that at some point, in a health or biology class, you learned how that works, so I won’t bore you with the regurgitation of those details). More than that, the state wants responsible, productive kids. Kids who will grow up to contribute the the growth of the Organism. Not kids who will grow up to mooch off the system, or commit crimes which injure the organism. So they come up with tax laws, and other perks to encourage people to get married, and to have children. Ideally, they want those perks to encourage people to stay married, and to raise their children responsibly (clearly the current perk system misses the mark on those points).
From the standpoint of pure biology, given that homosexuality is an evolutionary dead-end, it is not in the states interest to reward it, or encourage it.
Ah, but this is the 21st century, we can overcome that limitation, through advanced technology- in-vitro, cloning and what-not. Problem is that adds significant cost to the process of child creation, meaning it is that much harder to achieve a Return On Investment on the resulting human. It would be similar to a human eating nothing but celery. They would burn more calories in the digestive process than they would recover. In time, the state would starve itself for resources if it went down that path. (See Conservation of Humans for more info on this).
What about adoption, or non-technological, consensual fertilization? Those are certainly less expensive than the technological approach, but still more expensive than plain old vanilla heterosexual marriage. To thrive, an organism needs to be efficient.
And then there are the statistics. Numerous studies have concluded that children who are raised by both their biological parents tend to be better adjusted (mentally and socially), and have better overall health. This is compared to single parents, adoptive parents, grandparents, blended families... In all cases, children raised in a traditional nuclear family are better off. Yes, that is a statistic. Yes, there are plenty of examples of well-adjusted children of single parents, blended families, adoptive parents, etc… But, if you have a choice between betting a dollar on a 1 in 6 odds of winning $100, and betting a dollar on 1 in 2 odds of winning $100, you are going to choose the 1 in 2.
That is what the state is doing by rewarding nuclear families; betting on the best odds. It is seeking to improve the efficiency of the organism, and the probability of success. If anything it is in the states interest to adjust the reward system to more strongly favor the nuclear family (i.e. in the case of a divorce, the at-fault spouse loses all benefits, and is possibly penalized. In the case of an “irreconcilable differences” divorce, both are severely penalized. Children out of wedlock don’t qualify for dependent tax break status…).
The state can’t spend money just because we like something, or want something. The state has to invest on that which ensures the states survival. Otherwise the state will inevitably die. And by the state, I do mean “us”.
So, there you have it, you can breathe now.
A couple of those studies I mentioned, in case you want to see something, but don't want to break out google...