I think that the title of Achebe's work is justified in the character of Okeke.  Through his own evolution and change, the father ends up learning that marriage is, indeed, a private affair.  Okeke is quite insistent that marriage be seen as a public matter.  In the opening of the narrative, marriage is viewed as a public affair in how he selects a bride for his son and the insistence with which he pursues his son's marriage.  While he shuns his son in a callous and cold manner, he remains convinced that marriage is a public affair.

All this changes in the resolution of the story.  The letter from Nene shows that the marriage between Nnaemeka and Nene has been successful, and with children.  These children wish to meet their grandfather.  Nene's letter proves that the title is justified as she asks Okeke to meet his son and grandchildren.  For her part, she will remain in Lagos.  Such an action validates the private condition of marriage.  It is something that she will not sacrifice for her father- in- law.  Her asking him to meet his grandsons without her presence helps to justify how her marriage is and will always be a private affair.  Okeke realizes this when he contemplates his actions in shutting out his grandchildren, awaking with a fear that he might never have a chance to right his wrong.  This realization demonstrates how he has come to understand that marriage is a private affair, something that he has to concede if he wishes to see his grandchildren.  In this, the title of the work is justified as the characters have come to embrace its truth and validity.