If this occurs, there is already a problem, of which masturbation is not the cause, but a symptom. In any case, as quatona says, I would prefer not to base decisions about the ethics of such a widely practised activity as masturbation on the jealousy of a few spouses.
If my partner masturbated so much that I felt that I was neglected, that he failed to take an interested in me or my feelings in general, then there would be a problem. But that goes for anything: if he spent so much time watching football, or cooking, or with another woman, or with his friends, or going out dancing, that I felt our relationship was deteriorating, I would be troubled. Masturbation is no different, as far as I am concerned, and no more 'dangerous' to relationships than any of these things, unless it happens to be something by which you are personally bothered more than the others. And why would you be? I would submit that you would only be more bothered by masturbation than by other pastimes because of ignorance about the nature of masturbation and sex, and/or petty jealousy.