If you right-click a character and select the "Arrange Marriage" option in the diplomatic menu, then you just left-click to select that character. However, if you use the "Arrange Marriage" button in the character view (the one with two rings on it) to get a character list with suggested spouses, you have to use right-click in order to open the diplomatic menu with the marriage option already selected. Left-click in this case will open the character view.
While perhaps not entirely consistent. This is how it works at the moment. We will be looking at getting all the right-clicking/left-clicking to be more consistent in the future, though.
