To reproduce naturally a male must produce good numbers of normal sperm and be able to deposit them into the vagina of his partner during sexual intercourse.  Sperm are produced in the testes and pass into the epididymis, a structure on the side of the testis, for maturation, storage and nourishment.  During ejaculation, mature sperm then move into the vas deferens, empty into the urethra and are expelled from the erect penis during ejaculation. The volume of ejaculated human semen ranges between 2 and 6ml, depending upon the time since last ejaculation.

For natural conception there usually needs to be greater than 20 million sperm/ml in the semen, more than 50% should be moving in a rapid progressive manner, and more than 30% should have a normal shape.  Lesser parameters than these may indicate a problem and require resort to one of the assisted reproductive technologies to produce a pregnancy.
 
Semen defects are found in the male partners of around half of couples presenting with infertility.  A few such cases may have been inherited but a vast majority have no clear cause and treatments to correct problems are few.  It is thought that male fertility worldwide may be gradually decreasing.  The good news is that there are treatments available to make the best of whatever sperm are available and produce a pregnancy.
 
It is clear that healthy men produce the best quality sperm.  Overweight men, those who use tobacco and drugs, and those who work in potentially toxic environments are more likely to have reduced quality semen.