Sexual dimorphism
The things that make the two sexes of a species different are called secondary sex characteristics. They are not directly part of the reproductive system. They are the product of sexual selection for traits which give an animal an advantage. By "advantage" is meant:  more likely to stay alive and reproduce. In humans, beards are a secondary sex characteristic.  Human beings do not use beards to make babies.  A man can even shave off his beard and he will still be able to have children.  But men grow beards and women do not, and they are easy to see.