This is a translational issue and, depending on how Genesis 6:3 is rendered, its meaning changes greatly. For a Christian reader this matter of spirit or flesh (or both) is highly relevant.
Here is the same verse in three translations:
American King James Version
And the LORD said, "My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years" (my emphasis).
English Standard Version
Then the LORD said, “My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years" (my emphasis).
NET Bible
So the LORD said, "My spirit will not remain in humankind indefinitely, since they are mortal. They will remain for 120 more years" (my emphasis).
The first translation (viz., also) implies "he is also comprised of flesh" or that he is not only spirit. The second translation gives attention to the flesh alone, whereas the third emphasizes the mortality of the flesh. Other translations I've not included translate flesh as "he is corrupt" or something similar.
According to the translation, then, or even the context of the verse, what is being implied here? That man is also flesh, or that he is flesh, or that he is mortal, or that he is corrupt?
The following link shows the many different renderings which change the meaning of the verse: http://biblehub.com/genesis/6-3.htm
