Relationship's relationship record for the constituent also contains their email address from their individual constituent record. Selecting to export the email address of the relationship will display the data from their relationship record. The email address must also be listed there in order to export it from the 'Relationships' section of an Export.

 The email address does not automatically default on a relationship record because the Phone Type: E-mail is set to "Do Not Share" by default.  For the email address to default on a relationship record, the email address on the Constituent record would have to be set to "Shared".