For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world. Let's consider these three items individually. The lust of the flesh concerns itself with the cravings of one's bodily appetites. This includes illicit sex, drinking, drug addiction, smoking, or any other vice that the flesh practices. I am not preaching perfection.
Christians can slip and fall, but that is vastly different from becoming enslaved to fleshly lusts.
The problem is not one's possession of an old nature, but the old nature's possession of a person. Next, we discover that this new nature helps one to have victory over the cravings of the mind through the eye gate. Lust, via the eyes, is one of the greatest promoters of immorality today. This is why pornography and abominable sex movies are destroying lives, homes, and nations. They mentally undress every woman they see and vice-versa.