I don't think that masturbation by any partner, with porn, or just thinking about that cute secretary in the office, should be considered cheating. In a perfect relationship, you should be able to discuss this openly with your partner and work toward trying to share fantasies with each other.

Even President Jimmy Carter admitted to having "lust in his heart".

I do agree that hiding/lying is dishonest and wrong ... but I wouldn't call it cheating.