The term "flesh" is used metaphorically in order to contrast with the Spirit. And this kind of use makes perfectly good sense since Adam lost the Spirit of Life that was within him after he sinned. As many translations rightly render the word "flesh" to be the sinful nature. What is evil is not outside of us, it's what's INSIDE us that is totally depraved. And our skin is not within us, for your info.
Also our flesh (the skin that hangs off our bones) is not evil! To say that our skin is evil you would have to also believe that Jesus' skin was evil as well.
What a horrible, superficial, phony-baloney belief system you have if you actually believe that our skin is evil. Jesus clearly taught that what COMES OUT OF US IS WHAT DEFILES US! Maybe your skin comes out of you, but mine does not come out of me.
Therefore, just as sin entered the world through one man, and death through sin,
and in this way death came to all people, because all sinned— Rom 5:12
The reason our flesh is fallen, is that when Adam fell, so did his production of sperm.
The defect of the sin nature found in our flesh comes from Adam's falling.
It says? All sinned?
The Greek means, in one moment in time we all sinned.
For Adam sealed our fate biologically when he sinned!
And, on the other hand...
The ovum of the woman does not in itself contain the defect of the sin nature. Gen 3:15
That is why God was able to utilize the ovum of Mary to produce a sinless body for Jesus.
For the Holy Spirit provided the necessary genetic material needed to fertilize that ovum!
And, that also means? Every woman's ovum are free of the genetically defective sin nature!
For it's through the man's sperm that the sin nature is passed down.
Therefore, just as sin entered the world through one man...
That is true, even though the woman became the first fallen human!
Speculation time:
If Adam resisted and refused to eat?
And, had children with Eve?
Those children would have been born without a sin nature!
But, that never happened.