Thanks for bringing your question to JustAnswer.
In every relationship, there is a high need (for sex) and a low need partner. The low need person controls the amount of sex between the two of them, which leads to resentment by the high need partner.
The challenge is to make the sex more emotionally and physically satisfying--which will tend to make the low-need person more interested more often, and will be more satisfying to the high-need person (who may then lower his needs for frequency).
Some of the best books on this topic are:
1. How To Improve Your Marriage Without Talking About It. This book has the very best explanation of the difference in needs between men and women I've ever found.
2. Dr. David Schnarch has several excellent books about increasing the intimacy in a couple's sex life. Intimacy and Desire http://www.amazon.com/Intimacy-Desire-Awaken-Passion-Relationship/dp/0825305675/ref=sr_1_1?s=books&ie=UTF8&qid=1320150128&sr=1-1 and Passionate Marriage: http://www.amazon.com/Passionate-Marriage-Intimacy-Committed-Relationships/dp/0393334279/ref=pd_bxgy_b_img_b