X, you are almost right. It's not just about unprotected sex.
The thing is, that to religious Jews, it is considered a mitzvah (religious commandment) to have many children.
In many religious communities, especially in the Orthodox ones, they are saying "It is good to have many children, because God said so."
Women are taught that giving birth to children and raising them should be the #1 goal of their lives. Having a job is far less important. They don't teach them enough about the great responsibility in it. People in the community are often getting married at a very young age (20-24 for men, even less for girls), and immediately start thinking about offsprings.