Psalm 139:13-16 is speaking about when God places the spirit of a man within him. The flesh comes from dust. That's what Adam was made from, and we were all "in Adam". Procreation gives us a body from our parents. But our spirit comes from God. This body of death is conceived in sin, but the spirit is righteous.
Don't tell me you believe Psa 51:5 is saying we are all born sinful? That is not what it is saying, read it again.
Psa 51:5 Behold, I was shapen in iniquity; and in sin did my mother conceive me.
This verse is not saying David was born sinful. This verse is not even speaking about David, but about his mother. Only women "conceive", men do not conceive. A newborn does not conceive, the mother conceives.
And try this simple test, substitute other words and it will become obvious this verse is describing David's mother, and not David.
Behold, I was beaten in anger, and in fury did my mother strike me.
That may seem foolish, but it does demonstrate who the verse is really talking about, David's mother. Obviously she was doing something sinful when David was conceived, but we are not told exactly what. We do know that David's family treated him as a black sheep, and his father Jesse did not present David when the prophet Samuel requested to see all of Jesse's sons.
That's how we are all led stray - the spirit wages against the flesh. As each grows up, he becomes enamored with the things his sinful flesh desires, and he falls heading after them. To his own shame.
No, we choose to go astray, the flesh simply tempts us. There is a difference. Jesus was tempted in ALL POINTS as we are, yet without sin. He could be "touched" with the "feeling" of our "infirmities" (weakness). He felt the pull and tug of the flesh just as much as we do, perhaps even more, because he never satisfied the flesh when it would have caused him to sin. We give in to lust and have momentary relief. So, I believe Jesus felt the pull and tug of temptation even more than we experience.
Heb 4:15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
Sin causes death. If babies didn't have sinful flesh, then it wouldn't be possible for a baby to die. But in Adam, all die - even those who have not sinned in the likeness of his transgression
(see Zechariah 12:1-2, Ecclesiastes 12:7, Psalm 51:5, Romans 5:12)
Sin causes SPIRITUAL death, not physical. If Adam and Eve had eaten of the tree of life, they would have lived for ever, even though they were sinners.
Gen 3:22 And the LORD God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live for ever:
If sin causes physical death, then God would not have needed to separate Adam and Eve from the tree of life, they would have died anyway. No??
Think on that one.