If the question was "is doing it with a robot a sin", then answer would clearly be yes (I am using the English word sin in the context of imperfection). Adultery becomes a bit more complicated, and I am sure that a man and a woman would likely see the matter differently. Sexual relations between men and women involve many aspects. First there are physical differences. A woman can only normally make one baby a year, and that could be bad for her health since that does not give a lot of recovery time. A man can father several in a day for as long as he can produce sperm and not drop from exhaustion (I do know at least one man who was servicing several different women in one session for so long that he ended up in a hospital with a collapsed lung). This sets up some of the ways that one can view adultery. Historically, a woman is dependent on a man during child bearing and raising. From a man's perspective, a woman and children are an investment that consumes a considerable amount of what he earns with his work. Leaving out emotional and religious aspects, one could say that an action on the part of a man that reduces his duties and ability to care for the woman that he promised to care for would classify as adultery. And, a woman causing a man to have to fund and care for a child that is not his own would classify as adultery. Both are breaking a contract to some extent. In a monogamous relationship, both of the above would also both physically and emotionally be giving someone outside the relationship what rightfully belongs to the spouse. Perhaps this is why such actions are called "cheating" since one would be stealing what belongs to the spouse and giving it to someone outside of the relationship.
Where does a sexbot fit in to this? Assuming that it is a machine without emotion, and assuming that one does not achieve an emotional attachment to the machine, the issues above for the most part are resolved. The exception would be if a spouse preferred the sexbot to their spouse and robbed the spouse of the attention and affection that they owe them to be with the sexbot. I would personally consider this to be adultery. However, if my spouse was totally committed to me as a person, gave none of her affection to any other man, satisfied me in the manner required, but needed a 12 inch rubber penis attached to a five horsepower Briggs and Stratton engine (yes, these exist) to reach her own satisfaction, I would have no problem with this and would not consider her to be committing adultery. Likewise, if she was tired and worn out from the day's work and just wanted a rest, or was suffering from issues unique to women, and I made use of a Superwhore Mk III Android (I don't think these exist yet, but they are working on them), she would not consider me to be having an adulterous relationship (yes, I asked, but you can never trust the "sure, go ahead" given by a woman).
Personally, I think that it is more complicated than this, but it is a start.