Why does Okeke object to his son's marriage in "Marriage is a Private Affair"?

Okeke
objects to Nnaemeka's marriage to Nene because he is not following the Nigerian Ibo tradition.
Even though Nnaemeka has moved to Lagos (a big city), Okeke expects Nnaemeka to follow the
traditions from the Ibo village where he grew up. Instead, Nnaemeka meets Nene, falls in love,
and decides to marry her. First, Okeke specifically objects to the marriage because Nnaemeka
refuses to marry the Ibo girl that Okeke has picked out. Okeke also objects to the marriage
because of a couple of important specifics about Nene: she works as a teacher (which Okeke
believes is forbidden in the Bible) and is not of the Ibo tribe. It is this last fact that
absolutely infuriates Okeke. As a result, Okeke vows never to see or speak to Nene. Luckily,
Okeke's grandsons eventually cause Okeke to relent. Okeke plans to be reunited with his son and
grandsons by the end of the story.