How far do you think 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.