There may be a variety of issues behind this. As men age, their testosterone level decreases. In addition, if a man’s health begins to decline on account of obesity, lack of exercise or stress, his interest in sexual activity will decrease.

First, I recommend he have his testosterone level and general health checked by a doctor. It is not uncommon for male testosterone to begin declining around age forty. A woman’s interest in intimacy typically continues to remain strong.

Second, I encourage him to take care of himself physically by eating healthy and exercising. At a recent pastor’s conference, I was shocked by the number of pastors in the room that experienced heart attacks around age fifty. When I talked with them, most of them admitted to not caring for their health. Physical fitness will not just lengthen your life, it enhances a man’s interest in marital intimacy.

Third, your husband needs a biblical view of sex. This is a view where sex is understood as a God-given way to serve your spouse, not to serve yourself. He may not be interested in intimacy, but that doesn’t matter. If his wife is interested, it is a way he can serve her. He should make use of that opportunity.

The husband should give to his wife her conjugal rights, and likewise the wife to her husband. For the wife does not have authority over her own body, but the husband does. Likewise the husband does not have authority over his own body, but the wife does. Do not deprive one another, except perhaps by agreement for a limited time, that you may devote yourselves to prayer; but then come together again, so that Satan may not tempt you because of your lack of self-control. 1 Corinthians 7:3–5 (ESV)

Fourth, it may be appropriate to take a lesson from the Shulammite. When her husband was too preoccupied with work, she danced before him in a marital strip-tease to get his mind off his work and interested in intimacy with her (Song of Solomon 7). I am not necessarily recommending you dance! I am recommending you put thought into creative ways to recaptivate your husband’s interest.