Answer:
   
   Halachically, a woman is not obligated to keep her husband sexually
   satisfied. Men do have such an obligation toward their wives, but not
   the reverse. Theoretically speaking, this is an incredible
   perspective, totally unlike the cultures that Jews lived amongst at
   the time. So, in biblical times, what did men do in such a situation?
   Well, until a millenium ago or so, when Rabbeinu Gershom enacted some
   new legislation, only men and abused wives had the power to initiate a
   divorce. This meant that there was no way for a woman to get out of an
   unhappy but not abusive marriage. The courts would get involved in
   various issues to guarantee her happiness, including sex. Note also
   that a women refusing marital relations can be deemed "rebellious" and
   be fined from the money due to her upon divorce, to such an extent
   that the man will eventually be able to divorce her without any
   divorce settlement at all. This balances a man's obligation, to some
   extent. Also note that, in such a case, a real-world beis din would
   typically try to get them to go to counseling.
   
   Thus, it was assumed that if a man felt that he would be happier
   without her than with her, he would divorce her. Sex is a significant
   part of that--but not the only part. Further, it was a given that if
   the marriage was otherwise happy, the actual divorce wouldn't be
   necessary--knowing that her husband was miserable enough to consider
   it would be enough to motivate a loving wife.
   
   What should you do today if you are in such a situation. Here's some
   advice:
    1. Be careful never to imply to your wife that you thinks she's off
       kilter or a chemistry set. Not having her feelings taken seriously
       is a definite turn-off.
    2. See if there is a reason why she's not interested. Is she
       overtired or overworked? Do you ever do anything romantic when sex
       is not at issue? [For example, try surprising her with flowers on
       a day that happens to be during her time of the month, and you can
       therefore resist the temptation to "cash in" on it.]
    3. Perhaps she prefers being the pursuer than the persued. Since it's
       not working anyway, you can experiment to see what would happen if
       you drop the subject for a while.
    4. Try comprimise. Solving problems by comprimise is a key ingrediant
       in a successful marriage on the intellectual level as well.
       Totally unrelated to the sexual problem, an inability to see each
       others side would be itself a problem.
    5. Consider obervance of the halachos of taharas hamishpachah (lit:
       purity of the family; i.e., the laws about sex). This would
       guarantee her that for around 12 days a month the marriage will be
       centered on the head and heart, not the gonads. Deciding to try
       these halachos may be a good framework with which to begin. It is
       interesting to note that seven of these twelve days were not part
       of G-d's original legislation, or even a rabbinic enactment.
       Rather, they are something women of the early second Temple period
       took on themselves, and only subsequently became enshrined into
       custom and law. Perhaps they speak to a need inherent in female
       sexuality.
    6. See a counselor by yourself. Often, we lack the tools to change
       what we must. Ask someone for help is like stocking your toolbox.