We read a lot in the culture wars about marriage, like it is a concept that has been around forever and everyone knows what it is. That isn't quite true. Marriage as we know it today, is a fairly recent invention. As recently as 500 years ago, few people in Europe bothered to get married. Only the landed classes tended to formalize marriage. There are many cultures that have or had no concept of marriage. Others practiced very different forms of marriage (think Muslim and Original Mormons). Some cultures do not even have the concept or words for husband or wife. The kind of marriage practiced today is even different from that practiced by our great grandparents. One need only compare the divorce rates now and in 1912.
The ironic thing is that religionists, who complain most about the defending marriage, are leading the way in divorce and groups like Baptist ministers equal the general population in divorce. All this simply illustrates that marriage is neither stable or universal. The goal posts are always moving. We will see them move again, I am sure. With the marriage rates declining in most of the Western world, churches will need to admit unmarried people willy nilly, or they will have no members, just as they have had to do with divorced people.
See chapter 11 of Sex and God for specifics on several cultures. I am speaking at Lake Hypatia, AL on June 30. Then on to Reno, NV July 6-7 to speak twice at the National Mensa Convention the to the Bay Area Humanists July 8. Speaking calendar at IPCPress.com