Modern assumptions about marriage were created in the Middle Ages. Ancient Judaism had assumed a powerful man would have multiple wives and probably a few handmaiden/concubines, but Christianity settled on one spouse per person. But the idea of a Christian marriage really only developed in the sixth century.
Before then, Christians continued to follow Roman marriage practices, which assumed marriages were arranged by the male members of the family, and the wedding itself was a big, drunken party (we still have that part). The chief way for the Romans to distinguish a wife from a concubine (concubines were fine) was that a wife brought a dowry to the marriage, and a concubine didn't.
By the ninth century, Christian marriage settled into the form it would keep for the next millennium. Concubines were decided to be just wrong. Once married, one was supposed to be married for life. The only times divorce (really annulment) was allowed were when the couples hadn't "really" been married in the first place, if for example one of them was already married to somebody else, or they had been forced unwillingly into marriage, or, the biggie, they were too closely related.
"Too closely related," from the ninth century to the thirteenth, meant not sharing any ancestors seven generations back. Since one's ancestors double with each generation, in practice this meant that everyone in roughly the same social circles was related to everyone, and the aristocracy could essentially get divorce on demand by comparing family trees. Among the peasantry, however, no one really cared as long as the couple were more distantly related than first or second cousins.
Among the aristocracy, marriages were often arranged for political reasons, to cement alliances. Among the peasantry, economic considerations played a big part. But even if the couple's families had decided on the marriage, the couple did have to agree, and once married they were expected to love each other and stay true to each other. Even men, who it was assumed would fool around before marriage and if widowed, were not supposed to cheat on their wives.
Starting in the twelfth century, the romances were all about love, about people meeting a (socially suitable) person of the opposite sex, falling in love, and getting married. Not surprisingly, this ideal conflicted with what the parents had in mind.
By the twelfth century weddings were usually held on the church steps (not in the church itself), though marriage did not officially become a sacrament until the thirteenth century. The couple would give their oaths, before witnesses, to stay true to each other for life and exchange rings as a symbol of this. (This looks a lot like a modern wedding.) They would then generally proceed into the church for the nuptial mass, the first time a priest was involved.
But the couple was not really married before the "copula carnalis," actually consummating their union. If one skipped this step, it was grounds for annulment. Wedding guests might "help out" by shouting tips through the window after the new couple was tucked in together.
(Click here to find out about medieval divorce.)