Is another human being necessary for satisfying sex? Secretly—and perhaps especially on Valentine’s Day—many people love the idea of somehow being free from the constraint of needing another person to experience the heights of sexual excitement. But they shouldn’t get their hopes up: The state of the art in erotic gadgetry is still very crude, and there are reasons, both technical and deeply human, to doubt that we will soon transcend the familiar bodily requirements of our most urgent desires.

Sex robots now on the market...