Hi Waffles,
The issue of masturbation is taboo for many people so I can imagine that many people would frown on it being carried out even after being married. In principal, I don't think there is any problem with it being done in the context of marriage, especially if the person masturbating is fantasizing about his/her spouse. Now, the issue for me goes beyond whether it is right or wrong to commit the act. I would be curious as to why the person is masturbating, that is, pleasuring himself/herself, rather than spending the time and energy pleasuring his/her partner. Of course, there are certain specific circumstances that may cause this behavior: the couple might be physically apart from each other at the time, the women is menstruating, the woman is pregnant or just had a baby, etc. These situations would seem to make such behavior understandable, if not justified. But if the two are together under the same roof at the time, fully able to have sex with each other, and one is off in a corner masturbating, isn't there something wrong with that picture? It could be a sign that there are certain issues that the couple needs to sort out with regard to their relationship (which goes beyond their sex life).
So, in summary, while in principal I think it is acceptable, it could be a warning sign for other problems in the marriage.
God bless,
JR