I think that part of this answer exists in how the issue of caste is a public issue. Castes are constructed in a public manner. Social orders driven by caste help to align society and construct social expectations. Intercaste marriages pose a fundamental challenge to this condition of being. The intercaste marriage is a private response to a public design. It is a statement that the institution of marriage is private, while the caste system sees it as public.
It is for this reason that marriage is something that Okeke perceives as a public affair and something that Nene and Nnaemeka view as a private one. It is here in which the challenge is raised. Okeke uses the public notion of marriage as a way to try to convince his son that his marriage to Nene is not permitted. As he internalizes the public nature of the caste system, Nnaemeka essentially privatizes the issue. His argument to Okeke is that he cannot marry someone he does not love. His love for Nene is private in nature, and thus becomes a challenge to the social construct of marriage, something that Okeke and the caste order believes. The resolution of the story as one in which Okeke has changed indicates that while there might be social constructs to marriage, it becomes a private affair in that emotions and human fallibility assume a greater role than social design.