For what it is worth, my wife did her master's thesis on the evolution of marriage law in medieval europe. It has been quite some years since I was forced, errr, I mean gladly volunteered, to review it, but a few things stick in memory.
Most relevant to the above was that long after church law was well established, and the upper class's were following it, observers still commented that in peasant villages it was common for people to simply move in together, with "no" ceremony at all (I suspect that actually meant no church ceremony). Certainly this suggests that from a sociological point of view, it is entirely possible for the 'lower classes' to follow a completely different pattern than the upper classes.
In earlier medieval times, there was a lot of conflict between upper class traditions and law on the one hand, and church law on the other. Frex, in Germany by tradition and law nobles had concubines with recognized status, something forbidden by church law. Eventually church law won out, but it was no quick thing.
In Glorantha there is the fact that some of the laws are very literally handed down from upon high. However the biggest issue tends to be "what did our ancestors do." A secondary issue is that it seems divine law tends to be quite specific in some ways. So yah, just living together, having sex, and jointly taking care of kids may not mean that you are married, so it may not matter who you do it with.
Also, "what our ancestors did" is awfully important. I'm still not clear (or havent' decided which of conflicting opinions I prefer) how much blood line ancestors are kept distinct within a clan (so can different blood lines have different ancestral traditions, or are all subsumed within clan traditions).
Another thing that occurs to me regarding cottar marriages, is that in some cases they may be added as negotiating points in the marriages of thanes or important carls. So when the god talker Ernaldness marries into the Blue Fox clan, part of the agreement is that the Blue Fox find husbands for four cottar women who come with her and join her household.
--Bryan
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