A marriage is between one man and one woman. Ok, if we assume the only reason for marriage is to procreate, and multiply. But where does THAT assumption come from? Marriage is the manifestation of love between two people, but expressly for the purpose of reproduction? If those assumptions be the case, then logically it must follow that if a couple doesn't produce offspring, their marriage must be made null-and-void.

If we assume that marriage's most important reason is for procreation, then the length of one should be based on how long it takes to procreate. If in, say 3 years, you haven't produced offspring, your marriage should be, automatically on that next day, made null-n-void. At that point, you'd have to re-register to have the marriage re-instated. You'd have another 6 months to try, but by that point, if you hadn't produced kids, there's something wrong with one of you and you both need to find other partners, so it would seem.
I think now with all the starvation and overcrowding, and massive drain of Earth's dwindling resources, that marriages be limited to, like 2 kids. I mean that's only fair. We live much longer lives now, medicine and modern living keeps people alive much longer. Elsewhere people are dying of aids, cholera, wars, famine, drought, fires, whatever, but in the Western world, ie in this country, we're let to believe we live longer. If that's true, then this further negates the need for a big family. True there was a time when mankind needed to be "fruitful and multiply", but that time is no more. Everyone stuck on the freeway in "rush" hour gridlock of Interstate-95 knows the same thing: there's too many people on this planet

If the reason why gay marriage is "unnatural" is because two of the same sex can't produce offspring, therefore shouldn't be, terethen marriage stands as guardian of the future of our species. Marriage logically becomes the front-line, defining the terms of that expansion. By determining that marriage is one man/one woman, it has now, rightly become embroiled in contrevorsy, and combat. Through "defending" it, you are helping to destroy it.
Specifically it must weed-out all the unproductive citizens. If the intention of marriage is facilitate, legitimize the expansion of the species, then it does, truly have a unique place in our society. By branding it an "institution that needs defending", you are forcing it to take shots that it never should, if it is going to survive as a valid, vital "institution".
In defending the status quo you are using it to discriminate against all other forms of coupling. It is a good thing that it should be used to discriminate against certain types of people. If the object is to raise happy, healthy, safe children/families in a sane, desirable environment, then along with marriage, there should be included a mandatory child-raising training. If you fail that, you should not have children. Couples could still marry, but no kids till you pass the training.
It's only fair. Bringing life into the world is a huge responsibility. Shouldn't marriage be concerned about the lives it brings into the world, rather than just allowing people randomly to get married? Is marriage only concerned about the physical plumbing of squeezing a few more puppies into the world? Doesn't marriage have a responsibility to the children it's produced? It boils down to this: just because two people can produce offspring, doesn't mean they should. My examples for this are any Jerry Springer/Steve Wilkos Show, Divorce Court, Maury, and in fact, pretty much all daytime television.
In fact, from both my own personal experiences, and those of watching all those daytime "reality" shows, I've come to realize that marriage is a pretty f*ed-up, messy, yuckified affair. Marriage in it's current state is shabby, and defiled. It is this state that creeps like Brian S. Brown, Executive Director of National Organization for Marriage, are actively defending.
People don't need to get married. It's not like eating, or breathing, or a roof over your head. You can live, got to work, die just the same without ever getting marriage. But the female has been institutionalized, conditioned into believing that her only worth is that of attracting a husband-and-starting-a-family. This medaeval hanger-on, cling-on to our supposed modern, enlightened thinking isn't easily shaken.

Clearly marriage is indiscriminate, inanimate, passe, often irrelevant to real concerns of our world, our society. Marriage as it is is careless, and caused alot of damage in people's lives. The model of marriage as it stands is imploding.
Here's my reasoning on why that is. As a bi-guy, I've been with a number of married guys, more than all the gay men and women put together. To me that proves bi guys are attracted to other bi guys, but that's another discussion. The fact is these guys get to enjoy the both of "both worlds" as it were. Most wives will never find out about their husbands' extra-curricular activities. When he comes home, doesn't smell like errant kitty, he's good-to-go having passed the test...for that day. The reason I'm able to be with him is that, honestly, I'm easier to deal with me, another bi guy. It's also likely from the fact that I'm doing things with/to him that he can't/won't ask wifey to do to him. Sometimes it's the way that I'll rough him up, slapping him around like a true bottom-sissiboi. Obviously this is not something his wife can, or should know about. Me treating his as the true bottom, which we both know he is, is at once a very natural thing that we guys do to each other--rank each other in that relative Alpha male manner. Yes, he's cheating. But no, he's not cheating. It's only cheating if you're getting the same thing outside the relationship, that you're getting inside. The fact of the matter is that what I do with your man, you could, but more likely won't do with him. A woman can't substitute for a man, for a long period of time. If he's one of the 60%, then he's going to see me. If he's the other 40%, he's going to look for a female. Chances are that other female is going to do what the wife can't or won't do.

Marriage is caprecious: people fall in love, and people fall out of love. Once married, your real self comes out, and one can't escape what she or he has caught. Marriage without automatic, immediate deference to divorce is an anachronism. Statistics that I read have both marriage and divorce statistics side-by-side. Statistics can be used however, but the most interesting statistic is that people are getting divorced at about same rate they're getting married. (See: [link]).
There's also that great statistic entitled: Men take separation/divorce harder than women. An online stalker of mine was having a discussion with me about his recent divorce. His married experiences were helping him realize that he wasn't straight. His marriage soured him on women. One day he starts the discussion this way: "[Marriage] is so unfair for the guy." My immediate response was: "Marriage is unfair in general. It's a protected state for certain people." Apparently that's no less true in divorce: protected status for certain people.

All of this says nothing about those people who can't have children. Let's call these people by the name "transgendered". Maybe that's not a fair statement. Trannies are one of the subgroup under that rubric. Then there's people who've had operations/medical procedures, or ailments, or what have you that have rendered them sterile. Then there's the people too old to have kids. Then there's the people who have proven to themselves (and others) that they should never spawn again. Some of those people may have already produced offspring. If the couple is M/F, they're still allowed to marry.
And what about people who've been divorced 3, 4, 5, or more times. Shouldn't it have dawned on them that it's not working?

To summarize quickly: If marriage is anything, it's arbitrary, and capricious as it is. Marriage is destroying itself. Don't blame that on gays looking to be as miserable as those of you who have chosen a lifestyle riddled with problems.