A common tag used by several program in the FAM record is, _NMR meaning "not married" and can be used in place of the MARR tag.
It would look something like this in a GEDCOM:
0 FAM
1 HUSB
1 WIFE
1 _NMR Y
2 TYPE ?
EDIT: TYPE could be a one word term you think describes the relationship, "partners", "rape", "fling". I use these terms with caution, and many times use nothing because I can't describe it or don't know.