There are a few different Romance etyma with the meaning 'marriage'. Some are derived from Latin casa 'house', some from mater 'mother', and some from mas/maris 'man':
L casa [+ -mentum] > CA casament, ES casamiento, PT casamento, RO căsătorie
L mater + -monium = matrimonium > CA matrimoni, ES/IT matrimonio, FR matrimoine, PT matrimónio, RO matrimoniu
L maris + -aticum > FR mariage
For the origins of the word coming from "casa" in these languages, the word could literally be translated in to English as 'housement'.
Specifically, I have a hunch that the word would have originally meant the joining of two houses as this is what marriage typically meant in more traditional societies. However, I could be wrong and the house portion of the word could just refer to the household of the husband and wife. What is the significance of the root "casa" as a part of the word's etymology?