It is really evil to understand in an evil way. Let's just say that being able to enter and retreat refers to the mating process of human beings, and being able to enter and retreat refers to the male reproductive organs. The focus of this sentence is fine words, because we know that fertilized eggs need sperm and eggs to combine, so this supply refers to the process of sperm increase and storage during a period of no mating activity. As for the latter two sentences, it is actually a further explanation of this sentence.