Shared emails in a database
Many sites have taken to using an email address as the login id.  This works well for me as I don't have to remember the user id that goes with the email address used by the site.  This solution would require the couple to share a profile, if they use the same email address.   

Some mail sites allow adding a prefix following a punctuation character.  This gives email addresses like me.site@example.com or me+site@example.com.  This can be used to separate customers in the case like a husband and wife sharing an account.

Getting your data cleaned up will be a problem.  Instead of blocking users where there are multiple records with the same email address, you may want to offer to provide the user id(s) matching the email address.  I would send the id or list by email with a link that allows them to switch their id to the email address. 

Some sites allow or require an alternate email address.  It may not be appropriate to make that address unique.  However, you may want to monitor the number of accounts linked to the same alternate address.