Do you believe that marriage is a private affair?

I think that there are a couple of points here to make.  The first would be that globalization has had a far reaching impact to help make marriage and the discussion of marriage a more public affair.  In some respects, this is actually fairly beneficial, in that people are able to experience and understand different dimensions about marriage in more ways than before.  At the same time, when we define "public," one has to qualify "whose public?"  I still think that there are reactions and beliefs like Okeke's around the world.  There are some places where marriage is not a "private" affair between two people in love.  Rather, it is quite a public affair where individuals from far and wide either have input or are a part the marriage process.  Nene's attitude is critical in this process.  She understands that her marriage is going to remain a "private affair" from Okeke's wrath and his emotions.  Yet, she is willing to allow the public nature of it be explored with her children visiting their grandfather.  It is interesting to see how Achebe's ending reflects the whole notion of "public" and "private" in terms of marriage.  He cannot sleep, yet not out of excitement or jubilation.  He cannot sleep out of fear and "remorse—and a vague fear that he might die without making it up to them."  In this, one sees how marriage's public state, in terms of showing and saving face with villagers and people in one's community, ends up surrendering to the private one, whereby a father fears that he will not be able to make amends with his son, his grandson, and, to an extent, his daughter in law.  In this light, there is an element of age and mortality that undercuts everything.  Whether or not marriage is a public or private issue might not be the primary concern for Achebe.  Rather, he seeks to make the argument that marriage where there is love between two people and a commitment to make their world and our world a better place, cannot be looked at with scorn.  Whatever needs to be done in both the public and private realm should be done in order to ensure that little disdain is offered in such a relationship.  It is this need to educate the drives Achebe's primary motivation in the short story.

Is marriage a private affair according to the story "Marriage is a Private Affair"?

Achebe's story reveals how marriage is seen in fundamentally different contexts through the lens of generation and condition of life.  In the young and urban world of Nnaemeka and Nene, marriage is private.  It exists between two people that can come from different social settings and build a life between them distinctly separate from public judgment.  In Okeke's condition, marriage is public, a legacy that the elders hand down to the younger generation. Both visions believe that they are accurate in their understanding of marriage.  In the end, one vision has to give way to the other, or some type of negotiation has to be present because there is a competing reality between both.

I think that the ending of the story reveals that marriage is private. This is shown to a great extent. Marriage can be publicly brokered and remain a public property.  Yet, the sentiments of husbands and wives and how they live their lives is something that Okeke realizes he, as a member of the public, cannot control.  When he receives the letter about his grandchildren wanting to see him and reflects on the time lost, Okeke experiences immediate dread and fear.  He realizes that marriage is an institution that moves on without him. Fears of his own mortality enter into this equation.  Marriage is affirmed as a private matter because of the desire to see the family progress.  It is a private matter because Nene ends up controlling whether Okeke is able to be a part of his grandchildrens' lives.  The woman that he scorned and insulted becomes his key to the future.  The fact that she allows him the opportunity to connect with his past and future is what convinces Okeke that marriage is a private affair. While there are competing elements to marriage that are offered, the end result is that it is private because marriage is rooted in the emotional sensibilities of human beings.  The desire to publicize this and make it almost a commodity temporarily obscures this emotional reality.

How is "Marriage Is a Private Affair" based on events in the story?

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.