Okay, so, adultery in early medieval Scandinavia was a pretty simple thing, at least in Iceland. If one committed adultery, which is in this case the husband sleeping with someone else's wife, or the wife sleeping with anyone other than the husband, then divorce or fines (or, likely, both) were the options. Polygamy and open marriages weren't, so far as we have records, things that happened.

Roberta Frank's article "Marriage in Twelfth- and Thirteenth-Century Iceland" in Viator 4 is a pretty good summary of it, and, of course, you can always take a look at Grágás for the actual legal codes surrounding divorce and marriage.