Josh Rothman broaches the question, invoking the case of Korean hoteliers who tried to skirt prostitution laws by providing sex dolls for their patrons:
It seems inevitable, [David] Levy writes, that more advanced "sexbots" will push this issue even more to the fore, forcing lawmakers to figure out just which aspects of prostitution they want to outlaw. Levy's sexbot example is emblematic of a theme running through this collection of essays [Robot Ethics: The Ethical and Social Implications of Robotics]: The ethical problems posed by robots aren't just about the robots. They're also about old, familiar human behaviors which we must reconsider once robots are introduced. How will spouses feel, Levy asks, about the use of sexbots? Some will see it as adultery, others as a intrinsically meaningless.
(Photo from "The Love Doll," a series by Laurie Simmons)