One time some people in my colony named helen and john were going to get married and I had already had a marriage spot set up and they didn't even go to the marriage spot they just all the suddengot married right in my hallway. I don't have a save becuase when I tried to save it said I couldn't because an action was going on and so I missed and I didn't get to get a screenshot.
The marriage spot must have been unusable. Bad temperature, bad weather - there are various reasons this can happen.