I think some of the responses above have pretty much given you a good answer and would echo my opinion.
For myself, my wife and I have openly discussed our fantasies and inclinations and are free to explore them under the condition that it be with the other's knowledge, permission and together. Therefore, I have no need to do anything without her knowledge, but if I did, it would not matter if it was with another woman or a man, we both would consider it cheating and I would not do it.
So, my question I would ask of you and I think you need to address is why is it necessary to keep this from her? Are you ashamed in some way of doing something sexual with other men or are worried she will not respond well if she found out? Is there a reason you feel you cannot discuss this aspect of your sexuality with her and it could be something that could serve as fantasy fuel between the two of you? Are you worried she might react in a homophobic or disgusted way if you did discuss this with her or if she found out? Are you worried she would think less of you if you admitted you had even a slight interest in another guy's dick? Do you think her knowing this would be more or less shocking or unacceptable than if you told her you find other women sexually attractive?
As mentioned by others, if you are doing something sexual, even something you might consider minor like no-touching jerking, it doesn't matter the gender of those other people, you are keeping things from her and by continuing doing it in regardless is by its very definition, cheating.
But at the end of the day, it comes down to not whether anyone else considers it cheating, but whether you do. And it's clear by your moral compass, you do not. Since only you know both yourself and her, only you can answer why you even feel the need to keep this part of yourself secret from her to begin with.