A Boy and His Dog (1975) on Netflix

A post-apocalyptic tale based on a novella by Harlan Ellison. A boy communicates telepathically with his dog as they scavenge for food and sex, and they stumble into an underground society where the old society is preserved. The daughter of one of the leaders of the community seduces and lures him below, where the citizens have become unable to reproduce because of being underground so long. They use him for impregnation purposes, and then plan to be rid of him.