Let's face it: there are a hundred reasons for a woman not to have sex and you've probably heard just about every one of them. Lack of romance, stress, kids, schedules and on and on the list goes. There is one reason, though, that most guys never think of: her health.
That's right - a woman's lifestyle choices can lead her down a road of pain during intercourse, sexual dysfunction and debilitating disease. Yes, us men can do the same. In my one of my links, I discussed how important it is for a guy to be an Example of Good Health for the sake of his woman's health. On this page, I cover yet another reason (in a non-obnoxious, helpful way) to be a health leader: your sex life as a couple may depend on it.
On this site, I cover many of the lifestyle issues that can affect male performance and erectile strength. Below I'm going to cover some of the same issues for women: lifestyle choices that can sabotage her ability or desire to make love.
Here is just a partial list as to how health can affect her sexual function:
1. Female Sexual Dysfunction. Most guys think females don't want sex because of "emotional" reasons. However, very often it is related to something the medical profession calls "female sexual dysfunction". Female sexual dysfunction is correlated with all the same kinds of lifestyle issues that plague our male sexual function. For example, female sexual dysfunction has been found to be correlated with being overweight  and low levels of exercise . Another study states that "high blood pressure, high cholesterol levels, smoking, and heart disease are associated with impotence in men and sexual dysfunction in women". 
Again, this is all the same stuff that us guys struggle with and her sexual life depends on these every bit as much as yours. Often it has to do with "blood flow" issues and thus is somewhat "erectile" in nature. Blood flow to the clitoris and vaginal walls is critical for women as well. In fact, one rather suprising reason that this is so important is that lubrication in females is tied to nitric oxide production, a fact I discuss in my link on Female Libido and Viagra.
2) PCOS. This condition, generally from being overweight and/or having Metabolic Disorder, results in higher-than-normal testosterone levels. Sounds good right? Well, it's not: PCOS leads to ovarian cysts, infertility and has a common side effect of female sexual dysfunction as well. It is also a significant risk factor for heart disease.  Exercise and losing weight and other Metabolic Syndrome Solutions are options to discuss with your doctor.
3) Endometriosis. This condition, where uterine cells slough off and attach themselves to surrounding tissues, is also frequently linked with being overweight.  Endometriosis can be very difficult and often leads to infertility, pain and female sexual dysfunction.
4) Uterine Fibroids. These common and almost always benign growths can grow fairly large and cause heavy periods, severe cramping and other issues. In addition, they quite frequently cause an afflicted woman pain during intercourse. The underlying cause at this points seems to be estrogen. Dairy is actually a suspected risk factor by some experts, because of the natural estrogens present in cow's milk.
Obviously, this list could go on and on, but I chose these because they are so common. Let's start with PCOS: this afflicts between 5 and 10 percent of American women.  Endometriosis is still more common and occurs in 8-15 percent of women in their reproductive years in the U.S. Climbing the ladder, 20-40 percent of women have uterine fibroids of "significant size" and almost half of women between the ages of 50 and 74 report some kind of sexual dysfunction according to a Yale report. 
Again, encourage her (in a kind way) to exercise, eat right and take care of herself, or the odds start mounting - no pun intended - against you.
4) Gynecology and Obstrics, Apr 2009, 105(1):39-42, "Association of body mass index with severity of endometriosis in Korean women"
5) Arteriosclerosis, Thrombosis, and Vascular Biology, 1995, 15:821-826, "Coronary Heart Disease Risk Factors in Women With Polycystic Ovary Syndrome"
6) Menopause, 2009, 16(6):1188-1192, "Body mass index, urinary incontinence, and female sexual dysfunction: how they affect female postmenopausal health"
7) International Journal of Impotence Research, Jul/Aug 2008, 20:358-365, "Obesity and sexual dysfunction, male and female"
8) International Journal of Impotence Research, Sep/Oct 2007, 19:486-491"Mediterranean diet improves sexual function in women with the metabolic syndrome"
9) Urology, Nov 1999, 9(6):563-568, "Female sexual dysfunction: anatomy, physiology, evaluation and treatment options"