There is an option to select Partner, Friend, Single, Other or Unknown in addition to Spouse when specifying the relationship between parents.
There is a menu when adding a "spouse" where you specify a different relationship.
One the website, to edit the relationship:
Select one of the parents and Edit.
Select the relationships tab.
In the middle of the page to the right of the other parent's name it will say spouse. Click on Change next to Spouse and there will be a drop down menu where you can specify one of the other relationships.