The Right to non-discrimination on grounds of marital status is explained in these documents:

Legally Obligating Documents
African (Banjul) Charter on Human and People's Rights (3)
Convention on Elimination of All Forms of Discrimination Against Women (1)
International Covenant Economic, Social and Cultural Rights (2(2))
International Covenant on Civil and Political Rights (2(1))

Standard Setting Documents
Beijing +5: Further Actions and Initiatives to Implement the Beijing Platform for Action (68(g))
Beijing Platform for Action (93, 95, 106(g, h))
Cairo Programme of Action (7.3, 7.7, 7.8, 7.12, 7.14)
Universal Declaration of Human Rights (1, 2, 6)