Is same-sex marriage a civil right? No. But I don’t think different-sex marriage is a civil right, either. It is a practical matter.
The institution of marriage is a human invention. It started out as a property deal, for people with property. Poor people need not apply, though eventually the idea spread and “common law marriage” came to be, and even formal marriage. (Yes, the definition of marriage was expanded to include poor people.)
People do pair up, usually opposite sex, sharing property, and frequently having children. Sometimes there are problems, often they are quite familiar age-old problems.
This pairing up happens. It has consequences for society, it makes sense for society to recognize it and stamp a name on it, “Marriage”. Once it has a name it can be easily be referred to in laws and regulations–so much easier than dealing with each case on its unique merits, because face it: most marriages are the same in so many ways. It makes sense to take what we learn from one marriage and apply it to other marriages. Instead of having to invent laws that deal with things like divorce and child custody issues, from scratch, every time a marriage fails, it is only practical to send the feuding couple to a court that is already experienced with feuding couples, and sort it out with well settled case law.
One can imagine a world without the institution of “marriage”, but it seems an inefficient place. People are going to do some predictable things that have predictable consequences, it makes sense to have an institution ready for them.
Enter same-sex marriage. A few years ago it seemed ludicrous that there could be such a thing, yet now it is inexorably becoming obvious to everyone. Why? Because the closets opened and different-sex couples have become commonplace. They are everywhere and are being accepted as existing. (Like them or not, call them “normal” or not, they very clearly “are”.) And as with different-sex couples, it is merely practical to have a shorthand way to deal with them, too.
So dang it, let them get “married” it will make all our lives so much easier. It is a practical thing.
What about the slippery-slope worries? If we let same-sex couples marry, what about people marrying their horses? Well, if human-horse pairings become common, then we can figure out how to deal with that. In the meantime, same-sex couples are falling pretty neatly into the same pattern as different-sex couples, so let’s treat them the same.
It is not a civil rights issue, but it is sensible and practical and humane and fair.