I would say that the story goes very far in expressing how marriage is a private affair. In the final analysis, the intrusion of Okeke and the larger members of the traditional community do not stop the couple from marrying. Marriage was seen by the elders as a public affair. Okeke chose a bride for his son. Yet, this is something that Nnaemeka rejects as he chooses Nene. Both young people live their lives and while the father does his best to represent callous opposition, marriage is a private matter as the will of the larger community is not something to which the young couple capitulates as they live their lives.
The ending would demonstrate this. The fact that Nene is willing to put aside her own feelings in order to reunite father, son, and grandsons demonstrates how private marriage is. She is explicit about how she will not be there. She will maintain her own distance and revel in the act of the private while the public gesture is made. The outside intrusion that is initially demonstrated in the story is not one that defines modern marriage. Regardless of the claims of Satan or the will of the young to disobey the old as it allegedly says in the Bible, marriage is shown to be a private affair, between two people whose will to love is stronger than outside intrusion.