Because going on a low - calorie diet increases cortisol levels in your body. (dietselfhelp.com)
On the other hand if sexual activity means staying up late, disturbing our natural sleep patterns then then the positive effect will likely be canceled out by increased cortisol levels from less sleep. (predatornutrition.com)
Stress also increases cortisol levels, increasing hunger and affecting insulin levels. (progressivehealth.com)