What intrigues me is not the end result, but the process. I think it makes a strong case for emotional cognition, a field typically vastly underrated.
Or, put another way, the problem is not, in my view, solvable by reason. It depends entirely on an emotional and irrational judgment on whether "sex" with a robot is actual sex and, if so, what that should mean to questions of human fidelity. But the fact that resolution is intrinsically irrational does not make the question irrelevant or the answer obvious. To the contrary, when the technology reaches this point, answering this question will have profound implications for the shape an nature of society as a whole. But like many such things, it will depend entirely on reaching an emotional consensus about the issue.
For some reason, engaging in this sort of emotional consensus building gets a bad rep. Or, alternatively, people veer to the opposite extreme and decide that decisions can be made entirely devoid of reason. The notion that decisional processes may vary, and that different processes are appropriate at different times, does not set well with most folks.
But consider some examples. Why is the age of consent generally set at 18? While we can certainly find important behavioral/biological/judgment milestones at some ages, there is not much to distinguish 16, 17, 18, 19 or even 20. We have a zone of reasonableness and then set a consensus based on what "feels right." Yet this inherently arbitrary has real legal consequences -- as anyone convicted of statutory rape will no doubt attest. But to work within the zone of rational decisionmaking, we must indulge in some inherent arbitrariness.
Example of different modalities and their appropriateness. The Bible distinguishes quite clearly between "justice" and "social justice," and tells us when to employ one or the other based on circumstances. In the adjudication of disputes under the law, the abiding principle is a single and unwavering justice executed without regard to status. "Neither shall you favor the rich, nor the poor, justice justice shalt thou render." But in the legislative process and th formulation of policy, the Bible clearly directs people to exercise social justice of a very different nature. Mandatory tithes for the poor, strict limits on the enforcement of legal power (and if you shall take your brother's one night shirt for security, you shall return it to him in the evening, for it is his only garment), and a host of other rules that clearly elevate duty of one man over another in a manner utterly contrary to the Libertarian Western tradition purportedly found in Locke and Jefferson which leaves private charity to the individual and does not countenance injecting such concerns into the rule of law.
As individuals and as societies, we are capable of highly complex consensus processes using a mix of rational cognition and emotional cognition. But where one form of decisionmaking fails, as in the case of the morality of robot hookers,it does not trouble that we will end up relying entirely on the other.