I found this defintion in the UNDP's "HRM allowances and benefits dependency allowances" :
I would like to know if a relationship wih a boy/girlfriend or life partner (without being lawfully recognized) could be considered as a marital status."Basis for determining marital status
6. UNDP recognizes as a staff member's spouse any person who is lawfully married to him or her.
7. UNDP also recognizes legally recognized domestic partnerships, common law marriages and similar unions between a staff member and spouse that has been validated in reference to the law of the competent authority under which the personal status has been established, as per ST/SGB/2004/13/Rev 1.
8. UNDP’s acceptance of a person as a ‘spouse’ will not, in any case (common law or another form of union or partnership), preclude further review should a question of an existing prior marriage arise at any time. Nor will it conclusively determine the rights of competing claimants to entitlements as surviving spouses under UN or United Nations Joint Staff Pension Fund (UNJSPF) Rules and Regulations. "