Anyway, is it technically cheating if the robot has feelings for you, and you for it, and you have sex? Even though it is in reality only a machine?
Intuitively, I'd have to say yes. But I'm not sure why.
Thinking out loud here...
Firstly, it sort of depends on how two individuals in a couple define their union or their marriage. If both have agreed to engage themselves in a non-exclusive union, then of course sex on the side with a robot on the part of one partner is like sex on the side with a human, i.e., it's not cheating. But this is a trivial truth.
The harder problem is figuring out in what way a robot is like a human when it comes to sex on the side for a partner engaged in an otherwise exclusive union.
Secondly, and even before either a robot or another human enters the picture (or the bedroom), both partners need to see eye to eye on what it means for one of them to cheat on the other. Is it when an "outsider" brings you happiness or emotional fulfillment? Or just brings you off? Or both? Or neither? How many times do we see or hear of cheating spouses who seek to exculpate themselves by claiming their transgression was purely mechanical (so to speak)? "Honey, I swear! She (or He or, in this case, It) meant absolutely nothing to me!" when, in fact, it may most certainly mean something to "Honey." Those who are married but engage in these kinds of "meaningless" trysts or adventures may as well defend themselves by claiming they were only fucking a robot.
Finally, there's the prudish and outdated notion that, in any relationship, each of the two partners is to have exclusive domain when it comes to the sexual and emotional satisfaction of the otner. I've encountered couples that were ultimately destroyed by one partner having caught the other pleasuring themselves in their absence or having expressed a desire to indulge in fantasy games or to use sex toys and accessories. The slighted partner feels somehow cheated. "Am I not enough for you?" the complaint goes. Obviously not. This is a golden opportunity for partners to explore their sexual boundaries, but most couples that I know don't and won't. The inner core of who they are as sexual beings is to remain private. By all means, if sexual incompatibilities are too great to overcome, separation on the basis of irreconcilable differences seems almost inevitable. And there's no tragedy in this. When people are withering in a situation where they ought to be flowering, and no amount of couples therapy has had any impact at all, it's time to call it a night.
So.
Where am I going with this? Well, seems to me that whoever seeks sexual pleasure outside their union with a "human-shaped" artificial being is committing no great offense IF they know their partner would not be offended by them resorting to such a device. Who knows? They themselves could even get a sexual kick out of watching. BUT: if someone seeks out such a device in order to supplant (rather than complement) the emotional fulfillment they would normally derive from being partnered to their spouse (hence the supposition that this robot would have to be equipped with the ability to requite affection), then, yes, there's cheating going on.
I say all this, but I still need to think things through.
A fascinating question, Lotus.