The definition of libido means sexual desire.  The more technical definition such as defined by Carl Jung is more general.  He refers to libido, as the creative or psychic energy a person has to put toward personal development also known as individualization.

The male libido would be defined as male sexual desire.  There are times when male libido is decreased.  The causes can be psychological and/or physical.  If the male libido is decreased that does not always mean there could also be infertility.

The psychological factors that enter in to the male libido can result from such psychological causes as loss of privacy, loss of intimacy, stress, distraction or depression.  There may also be causes that come from environmental issues that create stress such as exposure to extremely loud noise over a long period of time or light that is extremely bright.  Some other factors that can cause stress are depression, stress, fatigue, being sexually abused as a child, sexual assault, trauma, neglect, issues with how they look, adultery, not being interested or attracted in their partner or anxiety over sexual performance.  

The physical factor that can be involved that may affect male libido is the testosterone level in the bloodstream.  This can be affected by various prescription medications, lifestyle and due to recent studies the physical fitness of their partner.

Lifestyle can mean being underweight or overweight, which cause low male libido due to lack of energy and abnormal hormone levels.

Medications can cause an imbalanced male libido especially if the medications are SSRI's, antidepressants, or beta-blockers.  There are certain cases of decreased male libido where impotence or any type of sexual dysfunction can be permanent.

Testosterone is a hormone that controls male libido and in certain cases where the testosterone level is increased, the level of the male libido is elevated.