What is the irony of the title "Marriage is a Private Affair" by Chinua Achebe?

The title is also ironic because it uses the word "affair," which can mean several different things. 

The word "affair" can refer to an incident or an occurrence. As the other educators pointed out, it can also refer to a private matter or concern. However, another meaning of the word is understood as, "an illicit relationship or dalliance."

The title states that "marriage is a private affair." Normally, no one would refer to a marriage...

The title is also ironic because it uses the word "affair," which can mean several different things. 

The word "affair" can refer to an incident or an occurrence. As the other educators pointed out, it can also refer to a private matter or concern. However, another meaning of the word is understood as, "an illicit relationship or dalliance."

The title states that "marriage is a private affair." Normally, no one would refer to a marriage as an illicit dalliance. In fact, marriage is a public declaration of fidelity. As is the usual practice, "forbidden" affairs are often conducted in secret, and the parties involved are usually not too keen on their secret being discovered. Much is at stake in such an arrangement.

In Achebe's story, the title indicates what is made clear in the story. The groom's father (Okeke) makes his disgust with his son Nnaemeka's impending marriage to Nene public knowledge in his community. The men from Okeke's community unequivocally sympathize with Okeke's grief and anger. They condemn in blistering terms Nnaemeka's rebellious insistence on marrying Nene. In fact, the men characterize Nnaemeka's actions as a grave sin against his parent and God.

In characterizing Nnaemeka's actions in this manner, both Okeke and his male compatriots have reduced holy matrimony between two people to the level of an immoral dalliance. In these men's eyes, Nnaemeka and Nene's marriage is not a sacred, lasting alliance but a wicked sexual flirtation that is bound to fail. In Nnaemeka and Nene's case, the irony in the title is clear: far from their marriage being a sacred alliance, it is now a "private affair," something to be ashamed of within the confines of Ibo culture.