Why do men lose interest in sex?

3. Infidelity. A man might refuse sex, citing fatigue, work problems, malaise, and increasingly engage in emotionless sex. Unfortunately, a woman cannot always recognize infidelity. But if, aside from these signs, there are also proofs of betrayal, it's worth contemplating the loyalty in the relationship. 4. General malaise, poor nutrition, lack of vitamins, etc. Illness and poor nutrition greatly affect libido.