Chances are that I am going to catch some flack for this, but I felt compelled to comments on this article which I saw on a friend's Facebook page. The article is entirely too long for me to respond to the whole thing, so I will not attempt to. Rather, I want to reply to some parts of it.
First off, the article gives a very good summary of the Catholic belief regarding marriage. The Church teaches that marriage is a sacrament with dual purposes. A valid marriage must be between one man and one woman and must be ordered towards procreation. If procreation is not intended by a couple, than the Church does not view the marriage as valid. Therefore, a homosexual couple's marriage cannot be valid in the eyes of the Church because it does not fulfill either of these purposes. Also, a marriage must be entered into freely (a coerced marriage is inherently invalid) and the couple must be faithful to each other. In addition, unless the marriage was invalid for some reason, divorce and remarriage are not allowed. Therefore, the Church can declare a marriage invalid (and therefore never a true marriage) if these requirements are not met.
All of that is very logical, but there is a major issue that arises from it. This definition is religious in nature and, as such, cannot be applied generally in the United States. The Founding Fathers (wisely) chose to not allow the state to form an official religion, thereby avoiding the interreligious and sectarian strife that so marred the Old World at that time and earlier. What this means practically is that the United States has set up a dual track for marriage. If a couple is so inclined, they can be married in a religious ceremony. However, to enjoy the civil benefits of marriage, the couple must apply for and obtain a marriage license. My youngest brother was married in a civil ceremony and not a religious one. My sister married a pagan and they had two marriage ceremonies in order to accomadate both of their religious beliefs. My point is that marriage is no longer solely a religious concern and, as such, cannot be limited by the beliefs of one religion.
After all, the United States is a pluralistic and secular nation. By that I mean what I said above, there is no official religion for the country. While a majority of the country is religious, there are a wide variety of different religions and there are also many people who are either agnostics or atheists. This means that no one religion can (or should) dictate how the country defines a public institution such as marriage. In a similar manner, the government cannot force a religion to accept a definition of marriage that differs from the one that the religion recognizes.
For civil purposes, there is no reason not to recognize a same-sex marriage. After all, there are many benefits (tax, inheritance, health, etc.) that come from being married. To deny these benefits to a same-sex couple is a matter of injustice in a civil sense. In a religious sense, there is a different story. Since each religion can define marriage on its own, different religions will allow different people to be married. Some religions will marry same-sex couples and others will not. There is nothing wrong with this and there is no legitimate reason for the government to force a religion to marry people if the marriage is counter to the religion's beliefs.
The article also addresses the sociological implications of marriage. I am not familiar with the literature, so I will remain silent on most of it, but there is one thing that I need to reply to. The article talks of the "traditional" meaning of marriage is that of one man and one woman. While this is generally true, particularly in Judeo-Christian influenced cultures, there are enough glaring exceptions to give one pause. There are many cultures where a man or woman can marry multiple people and where, in fact, this practice is undertaken particularly by the rich. After all, the richer you are, the more wives (or husbands) you can support, thereby making multiple spouses a status symbol. The article does note that there are people who favor marriage to multiple partners, a view which is not incompatible with history. While it may be looked upon as odd, if every member of a group freely agrees to be married, there is no reason (in a civil sense) to deny them the right.
That being said, I do think that we need to come up with a definition of marriage that will make sense. I believe that in a civil sense, the following requirements make sense:
 The marriage must be entered into freely by adults. Arranged marriages, marriages obtained through coercion, or marriages involving minors (with the exceptions listed below) would not be valid.
 As a corrolary to number 1, a marriage involving a minor would only be valid with parental consent and if the minor clearly and without coercion agrees to the marriage. Parental consent can be avoided if there is a legitimate and demonstrable concern that obtaining their consent will cause harm to come to the minor. In that case, the court will appoint a suitable guardian who will ensure that the rights and concerns of the minor are being looked after properly.
 If there is to be a marriage with more than two partners, then each member of the group must freely and without coercion agree to the union. Without that consent, the marriage would not be allowed.
 A religion would not be required to marry anyone when their religious beliefs are against the marriage. This would be a rule with no exceptions and there would be no civil penalties attached to any religion or church that refuses to marry people due to their religious beliefs.
I think that these requirements make sense and should be enough to protect religions and preserve their autonomy. They will also allow the government the flexibility to recognize a variety of different marital arrangements while simultaneously protecting people from being exploited or forced into a marriage that they do not want.
[Added October 21, 2013]
On my drive home after writing this, I was considering the multiple partners aspect of this and felt that I wanted to write a little more about it. Basically (as far as I know), there are two main views (not sure if this is the right word) at play here. The first is polygamy which is when one person is married to multiple people of the opposite sex. When it is one man and multiple women it is called polygyny and when it is one woman and multiple men it is called polyandry. The second is polyamory which is when a person has deep, intimate (although not necessarily sexual) relationships with multiple people. This can take the form of a group marriage or polygamy of some sort or it could be a less formal relationship. The thing is that in order to engage in a polyamorous relationship, one must practice radical honesty.
Polyamory is distinct from swinging or some open relationships because it is not about sex, rather it is about forming deep emotional bonds with multiple people with the knowledge and consent of your partner(s). Now, it is possible for these relationships to be either same-sex or opposite-sex and it is also possible for these relationships to be sexual, it would depend on the rules as agreed to by the partners in the relationship.
The relationship between this and my earlier post is that the writer of the article mentioned above was worried that if same-sex marriage was allowed could it be used to allow for polygamy or polyamory? To which I ask, why not? If it is purely a civil matter, if all participants are freely consenting adults then it doesn't matter. Also, as I said above, throughout history polygamy of one sort or another has been the norm. Monogamous relationships have historically been mainly an obsession of the Judeo-Christian European world. Is it wrong? No. But it is also not the worldwide, traditional version of marriage and should not be painted as such.
[Added October 28, 2013]
Saw this article over the weekend on CNN.com and thought it was interesting. Quite relevant to what I added to this post.