This is what the Constitution says about marriage in Article 45,marriage (n) 1. the legally or formally recognised union of two people as partners in a personal relationship (historically and in some jurisdictions specifically a union between a man and a woman) - Oxford English Dictionary
(2) Every adult has the right to marry a person of the opposite sex, based on the free consent of the parties....(4) Parliament shall enact legislation that recognises—(a) marriages concluded under any tradition, or system of religious, personal or family law; and
(b) any system of personal and family law under any tradition, or adhered to by persons professing a particular religion,to the extent that any such marriages or systems of law are consistent with this Constitution.
The phrase "consistent with this Constitution" is interesting to me. I link it strongly to another provision in the Bill of Rights, Article 27, which states at clause (4) that,
(4) The State shall not discriminate directly or indirectly against any person on any ground, including race, sex, pregnancy, marital status, health status, ethnic or social origin, colour, age, disability, religion, conscience, belief, culture, dress, language or birth.
My copy of the Constitution uses the expression "shall not" one hundred times. You will notice that in Article 45(2), the expression is not used. The right protected by Article 45(2) is that of two adult heterosexual humans to freely consent to a marriage between them. Article 45(2) does not state that an adult shall not have the right to marry a person of the same sex. The power granted to Parliament in Article 45(4) is to make laws recognising two kinds of marriages: those concluded under any tradition, or system of religious, personal or family law, and any system of personal and family law under any tradition, or adhered to by persons professing a particular religion with one caveat, that is, so long as the laws are consistent with this Constitution (which I have linked to the non-discrimination clause in Article 27). Parliament has not been given the power under Article 54(4) to declare certain marriages or certain systems of personal or family law to be unrecognised or unlawful. (It is likely that if Parliament made such a law, an interpretation of Article 27(4) could be used to challenge that law in the High Court.)
We claimed that our Constitution is one of the most progressive. One day we may be called upon to prove it.