Simply go to one of the people's 'Partners' tab, click on 'Edit partnership details' and select 'Married' from the scroll menu.
If the people in question have been set to other or no partners go to the 'Personal' tab, click on 'Change or delete', then on 'Change Partner' and finally on 'Set partner to [insert name here]'. This will make the line connecting the two people bold, symbolising they are current partners.
Hope this helps!