1) He is not deriving sexual pleasure. OP specified that he is doing it at work, and not is not being stimulated. In a direct sense, he is not deriving sexual pleasure. In an indirect sense...if we start talking about indirect ways to derive sexual pleasure, we will never stop.

Also, you do not get to decide what is infidelity. A marriage is a private contract and the parties to it decide what constitutes a violation of such contract in conversation with one another. OP apparently draws the line at physical contact (read her first sentence). But at any rate, it is something to discuss within the relationship and not outside of it.

2) I am surprised you would write that, because even though I am not into BDSM at all (emphatically so), as a sideline observer I have been able to learn that there are all kinds of BDSM relationship spanning the whole range from what you describe about your own relationship to that news report from Britain a few years back - a young woman was being walked on a leash by her dominant boyfriend. She was not allowed on a public bus, spurring the controversy. So again (I am saying that as an observer) - relationships are private contracts and people decide between each other what they should do - whether they should go out with the girl on a leash, or, whether they should go out presenting outwardly as a regular couple, but in bed she would enjoy his cruelty. It is just a contractual arrangement - that is all. If there are no victims, everything is OK (in theory, at least - if I were to actually SEE a young woman being walked on a leash, I would probably not be able to keep my cool and say that everybody gets to decide for themselves).