Zohar, Bereshit 61b
Rabbi Yitzhak was in the presence of Rabbi Shimon. He asked him: Regarding the verse, "And the earth was corrupt before G-d," (Bereshit, 5:11,) if men transgressed, why should the earth be called corrupt?
Rabbi Shimon answered him, Because it is written, "For all flesh has corrupted its way." In a similar fashion, it is written, "And the land was defiled, therefore do I punish its iniquity upon it," (Vayikra, 18:25.) If mankind sinned, why is the earth to blame? The reason is that mankind is the essence of the earth. When they pollute their way in the world, the earth becomes defiled. This is proven by the verse, "And G-d saw the earth, and behold it was corrupt, for all flesh had corrupted their way upon the earth."
Come and see, all sins of man, and all of his acts of defilement, can be rectified through t'shuva (repentance.) But the sin of spilling one's seed on the earth, which defiles the man and casts semen onto the earth, defiles both him and the earth. Of such a man it is written, "The stain of thy iniquity is before me," (Yirmeyahu, 2:22.) It also says, "For Thou art not a G-d that hath pleasure in wickedness, nor shall evil (ra) dwell with Thee," (Tehillim, 5:5,) except through a great t'shuva. On this it is written, "And Er, Yehuda's firstborn, was evil (ra) in the sight of the L-rd, and the L-rd slew him," (Bereshit 38:7.)