Go to the Person view for one of the parties and look for the Relationship icon in the green line. It is next to Timeline. Select Relationship and go to the marriage relationship. In the right hand pane you will see a field labeled relationship which normally contains spouse. Display the dropdown list and select the relationship you want to use, in your case Partners. I use Friends for a short term relationship. Some screens in FTM will still say Spouse but all reports will say they met, not that they married, and will use the relationship you selected. If you create charts be sure to include the Partners and/or Friends fact as one of the items to be included.