According to researchers, the adverse reactions can appear in humans because the nervous system of adolescents is still developing.  As the nervous system develops, its responses are in a constant state of flux and some of those changes, according to this study, are preparations for adult reproductive behaviors and physiology. 

In this stage, the brain can interpret sexual activity as a stressor, which could negatively affect brain development and make males more susceptible to symptoms of depression. 

Source: Science Daily