They still live for the most part in one room houses with the kids sleeping next to the parents. Not exactly conducive to sexual intimacy for mom and dad. So dad goes to a spa or sex club to be serviced. I know doctors who are chairmen of medical departments (so pretty high salary) whose entire house is the size of my family room. That is tough way for a family of 4 or 5 to live. It has to be worse for those with lower income.
And yet, that's the way the majority of people throughout history have lived, while rearing large families. Modern Japanese, if reasonably well off, have 4 or 5 room houses/apartments, which is more than my grandmother's family, in Missouri in the early 20th century, had: Parents and five children in 3 rooms. And they were a smaller family, for the time.