Do I think it's ok for both partners to masturbate? Sure as long as there isn't some issue with compulsive/addictive behaviors.
I do think that if you are involved in a relationship where one partner wants it much more than the other there are probably some underlying issues and concerns that need to be dealt with, and discussed, if not I could see that it would become the elephant in the room, and widen the gap in communication.
So if your spouse is the undersexed partner in your relationship you need to explore all the reasons for this. I would ponder these questions. I do not expect you to answer these on this forum....
1. Does she truly orgasm from intercourse? If not does she reach orgasm in other ways? If so do you provide those ways when you are intimate?
If not what is holding you back?
2. When you have intercourse, and intimate time together is she relaxed, and able to not worry about all the other problems in life, from kids, to chores, to job stuff?
(This was a big problem for me in enjoying our time together when the kids were small there was just too much to do, and worry about. The biggest turn on for me was to take some of that burden away, like coming home from work to find dinner made, and the laundry done, oh baby, he was gonna get lucky on those nights).
3. Is she terribly inhibited? Afraid to be seen naked? Doesn't like, or won't consider porn, as a method of getting the engines warmed up? Is there any use of toys or assistive devices like vibrators to also help?
If she is then I would urge you buy her these things, and let her know that you are not judging, and her pleasure is much more important than yours.
I can honestly say, and am proof that the better the sex the more I want it, and when it's not so great, the less I want it. We had a pretty good sex life throughout our relationship, granted there were times when the kids were small that it dropped off to one time a week, but I can tell you since Dday, and R we are quite active in the bedroom, including things that involve toys, mutual masturbation, and of sex.
There should be no shame in masturbation, it is a natural urge, we are so sexually repressed in American society that women are really reluctant to partake in it, but it can be really freeing, and often provides a great release from stress, and life.