The first married couple on The Land were Connor and Brigid, though we know from their conversations with God that the concept of marriage exists on virtually every other world in the Universe. However, this concept works in different ways on different worlds, and isn't necessarily even between just two people. (See polyamory.) Thus far on the Land, there have never been marriages directly involving more than two people, though there's no reason to assume such a marriage couldn't happen in the future. We also know that there has sometimes been debate on other worlds as to whether marriage should be reserved strictly between different sexes, or allowable between members of the same sex. On the Land, this question doesn't seem to have occurred to anyone for about the first two and a half centuries of our planet's existence. The issue is addressed in the entry dated 11 Sp'yet, 251, in the Book of Julia, in the O'Gas. Since that time, it has been almost universally accepted on the Land that same-sex marriages are entirely natural.
It is also known that on some worlds, marriage has been considered a matter of concern to both church and state, which each need to be involved in the actual joining of two people, in order for the marriage to be considered "official." This stands in stark contrast to the nature of marriage on the Land (at least until the passing of the adult license law). There could be any number of reasons for this, including the fact that Landian psychology is slightly different from the psychology of people on other worlds. In fact, on other worlds there have sometimes been people who have gotten married for reasons other than love, or even when they believed themselves to be in love, married without a sufficient amount of thought and discussion of the matter beforehand. ...In spite of the fact that the legal system is not involved in establishing marriages, Landians do add the Terran suffice "in-law" to relations through marriage. (That is, the parents of one's spouse would be one's "mother-in-law" and "father-in-law.")
As a side note, it is also understood that on some worlds, people choose to engage in sexual relations prior to marriage, a thing which is (currently) virtually unthinkable on the Land. While obviously people may be attracted to any number of other people, and have active sex drives, and certainly they enjoy dating prior to marriage, and engaging in a certain level of physical intimacy- kissing, etc.- Landian psychology makes the idea of having sex without being absolutely certain that you want to spend the rest of your life with someone... more disturbing than appealing. This difference in Landian psychology is attributable to the punishment incurred by our race for The Fall. While many people from other worlds might expect this punishment (enhanced loneliness) to make people more apt to get together without necessarily being completely in love, the reverse is actually true. The actual nature of our loneliness is a feeling of separation, of not really understanding one another, not belonging. For this reason, it is of the utmost importance to find someone with whom you feel very strongly that you do belong, before being comfortable enough with them for such a deep level of intimacy.
And so, when two people have known each other a long time, and put a great deal of thought and discussion into their feelings for each other, they may eventually come to the realization that they do want to spend their lives together. At that point, they make private vows to each other (which are understood to be largely similar to marriage vows on many other worlds), and from that point on, they consider themselves to be married. And naturally, anyone they inform of their marriage accepts this, without question. It is of course possible that at some point in the future, one or both members of the couple may find that their feelings have changed, and the marriage may be dissolved as simply as it was formed... though again, not without due thought and discussion.
It should be noted that while elves and merfolk are not subject to the punishment of the Fall, and therefore have different psychology than humans, both races tend to have similar attitudes concerning love and marriage as do humans, though it is likely that they may be slightly more casual, in some cases. Little is known by land-based society about the nature of merfolk society, though it is known that most elves tend to choose life partners at a younger age than do humans.