I am sure that you are well aware that this brilliant short story deals with intercultural marriages in a radically different context than India. Achebe's fiction focuses on Nigeria and one young man's decision to marry out of his Ibo tribe and the catastrophic impact that this has on his relationship with his father. Achebe in this story essentially contrasts two views about marriage, one that is older and traditional, which states that marriage is not a private affair and is the responsibility of the entire tribe, and one that is more modern which argues, as the title says, that marriage is a private affair and is the choice and decision only of those involved. The way that Nnaemeka's father shuns his son for so long because of his decision to marry outside of the tribe shows how powerful concepts such as tribe and caste still are in various parts of the world, and the very real sacrifices that people make who chose to ignore these forces.
However, the overwhelming message of the story is the way in which holding on to strict notions such as tribal identity and caste in this area can actualy be profoundly damaging to us as we miss out on relationships because of the way in which we cling on to antiquated notions. Consider how the father responds when he finds out about the existence of his grandchildren:
How could he shut his door against them? By a curious mental process he imagined them standing, sad and forsaken, under the harsh angry weather--shut out of his house.
That night he hardly slept, from remorse--and a vague fear that he might die without making it up to them.
The cost of shunning your son because of his choice of marriage partner is made staggeringly clear, as the father is left desperately regretting the time he has wasted which he could have spent with his grandchildren.