I've got an imaginary soap box, and I'm going to use it.
One of the arguments that I hear used as justification for defining marriage as "one man and one woman" is that the whole purpose for marriage is the production of, care of, and setting a good example for offspring. That particular argument gets my fracking goat in a much bigger way than people who argue based on their religious beliefs.
Religious beliefs are all well and good, but I'm talking about the government and legal rights here. And people who use the argument that procreation is the foundation of marriage are often doing so to appeal to people they know aren't going to buy into the religion argument.
I've got gay friends who are raising children without the benefit of the legal rights a marriage license would provide. Surely, under the "marriage is for the family/children argument", they have more right to legal marriage than I do.
Some of my gay friends who have children were married during the all-too-brief window when marriage licenses were available to homosexual couples in California. Some states now recognize their marriages. Some states don't. Some never did. Mommy and Mommy - or Daddy and Daddy - are married in part of our country but not in other parts. What does that say to their children about the importance of the institution of marriage? Seems to me that if we really valued marriage as a civil institution, it would be too important to allow it to be tainted by discrimination. *
To take the argument to an extreme, if society's need for bearing of and providing a stable environment for children is the whole reason for marriage, I suppose I should have divorced my Mister as soon as I was medically no longer capable of producing a child. Here he is, still of suitable breeding age, and legally bound to a woman who can't validate the union by bearing him any babies. What kind of a marriage is that? Surely I should be ashamed of myself as a citizen, since I'm preventing him from contributing to the population, as is his duty to the nation, and to his genetic line.
At very least, I suppose, we should all be forced to sign an agreement to either breed or adopt within [X] time period, in order to qualify for a marriage license. And if that sounds ridiculous to you, let me say that denying loving gay couples the legal protections for their union that straight couples are allowed seems equally as ridiculous to me.
*Remember, I don't care if your religion recognizes their marriages (or mine for that matter) as valid. Our government currently regulates marriages. Arguing about semantics just delays the correction of an injustice. It is just not going to happen that you're going to "get government out of the marriage business" any time in the foreseeable future. So lets focus on what we do have - which is marriage licenses - and get us a step closer to that dream of "liberty and justice for all". Legalize those marriage licenses for gay couples - just as they're legal for elderly straight couples; 80 year old men marrying 20 year old women couples; straight couples who don't intend to have children; straight couples who are obviously headed for divorce but they're getting married anyway; and myriad other sets of straight couples who don't fit the profile of the perfectly mated breeding pair of conformist citizens.