The Bright Companion

From the back cover:

When the by-product of the greatest contraceptive ever discovered turned out to be sterility for the next generation, the result was predictable. Within a century the population of the 'civilized' world dropped to a tiny remnant of narrow-minded survivors. Their few communities were ruled by hard-shelled fanatics whose prime need was women who could still bear children.

Anne had been such a trade-off "wife"and when she slew her captor and fled, there was only one who would help her. This was the young man whose hidden stock of medicines made him a necessity to the isolated homesteads.

The surprising and exciting adventures of Anne and David as they fled their barbarous pursuers and headed for a legendary sanctuary far across the ocean is a realistic and imaginative projection of a world that yet may be.