What is the probability of producing kids by mistake?

I do not really understand it when some parents say they gave birth to a child by mistake, it sounds odd to my ears and it is something I do not seem to understand.

credit

I have a family friend who is just four and she has four other siblings, how the mother has the able to cope with the process of giving birth and as well training the children is simply wonderful to me. The excuse the parents give after the birth of each child is the fact that the pregnancy was a mistake.

My dad was speaking with his friend over the phone one day and he screamed. She is pregnant again?
The man did not have money to even take care of the three kids he had on ground as I have heard him complain bitterly on several occasions about the fact that his three children have nothing to eat, the other day the wife got pregnant, he called to cry that it was a mistake and he had no idea how it happened, it was something he could not just explain and his friends had promised to support him any way they can to train the child, that was some years ago.

credit

This year again he had called to say that a similar mistake has reoccurred, his wife had mistakenly gotten pregnant for him, he had no idea on what to do because he could not lay the life of his wife down on the line.
But the issue is, why there is always a recurrent mistake all the time, are there no ways to avoid getting your wife pregnant all the time.