We are only as good as our nature
The reason we have sinful inclinations, suffering etc. to begin with is due to our nature as fallen human beings. We are fallen and not 'perfect' human beings because Adam was created specially; but we are born from him, and are therefore according to his nature as we recieve it. Since the Fall corrupted human nature, it was passed onto us as it was in Adam.
We read about this in Genesis 3. Close attention must be paid to gather all that is being conveyed in this narrative. No matter how much of it you interpret as allegorical. (I don't take it to be allegorical at all).
6 And the woman saw that the tree was good to eat, and fair to the eyes, and delightful to behold: and she took of the fruit thereof, and did eat, and gave to her husband who did eat. 7 And the eyes of them both were opened: and when they perceived themselves to be naked, they sewed together fig leaves, and made themselves aprons. 8 And when they heard the voice of the Lord God walking in paradise at the afternoon air, Adam and his wife hid themselves from the face of the Lord God, amidst the trees of paradise. 9 And the Lord God called Adam, and said to him: Where art thou? 10 And he said: I heard thy voice in paradise; and I was afraid, because I was naked, and I hid myself. 11 And he said to him: And who hath told thee that thou wast naked, but that thou hast eaten of the tree whereof I commanded thee that thou shouldst not eat?
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To the woman also he said: I will multiply thy sorrows, and thy conceptions: in sorrow shalt thou bring forth children, and thou shalt be under thy husband's power, and he shall have dominion over thee. 17 And to Adam he said: Because thou hast hearkened to the voice of thy wife, and hast eaten of the tree, whereof I commanded thee that thou shouldst not eat, cursed is the earth in thy work; with labour and toil shalt thou eat thereof all the days of thy life. 18 Thorns and thistles shall it bring forth to thee; and thou shalt eat the herbs of the earth. 19 In the sweat of thy face shalt thou eat bread till thou return to the earth, out of which thou wast taken: for dust thou art, and into dust thou shalt return. 20 And Adam called the name of his wife Eve: because she was the mother of all the living. 21 And the Lord God made for Adam and his wife, garments of skins, and clothed them.
In defying God and doing what it was they were commanded not to do (however much of an allegory the forbidden fruit is), several things happen:
Their nature, clearly, is changed, inasmuch as they now recognize their being naked is shameful and feel the need to cover up and hide from God. This is both real, and yet symbolic of yet a further inner shame and realization of the gravity of sin and evil, in which they've just now taken part.
Women and men in mankind now recieve the punishment of toiling and being in pain. Examples relevant to the sexes are given (giving birth; working the field), but are not exclusive, due to their obviously summary nature.
This kind of change in nature is not something which will not be proper to their children. It is an intrinsic change in what seems to be more detectable or manifest in the corporeal component of their nature as a body-soul composite (Genesis 2:7 cf. Ecclesiastes 12:7).
You can't have taken from you what you never had
Actually, it is fair. We recieve a nature different to that which Adam and Eve originally had. A fallen human nature, to be precise. As such, we were never entitled to—just as they weren't after they sinned (or technically even when they had it: Romans 11:35; Job 41:11)—to the life of grace in which they had originally been constituted: amity with God, enjoyment thereof in general, and all its benefits.
This is the great loss of the Fall—loss for Adam and Eve, but not for us! You see, we never had it. Nor are we entitled to anything, let alone paradise. We are expected to obey the inner voice of God, in our conscience, calling us back to Him, ultimately, through Christ, who can restore us to that grace in baptism; the Church teaching that Mary was the firstfruits of that Redemption, being not freed from original sin, but consituted as a new Eve to the new Adam, Christ (1 Corinthians 15:22,45), without ever having had it—an Eve for a new creation, in which the devil doesn't decieve the woman, but is conquered through one (Genesis 3:15).
The only unfairness would be if we were entitled to and originally possessed some greater happiness, and then had it taken by violence. Except we have only been what we are: fallen human beings. Since it is not impossible to make it back to God and recieve divine life, grace, again, it is not unfair at all.
In addition, we also partake in Adam's sin in a real sense, each time we sin. To sin is to tread on the same forbidden ground that Adam did. Because any sin by definition is willful disobeying God, we commit the same kind of offense as Adam, to varying degrees, with every sin. You could say Adam's sin was to will to sin period.
Romans 5:12
Wherefore as by one man sin entered into this world, and by sin death; and so death passed upon all men, in whom all have sinned.
When Adam sinned, as it were, we were all there, sinning with him. Because our having sinned is known with God from the beginning, and ourhaving sinned is to partake in the same sin of Adam. But our personal sins are not our original sin. Original sin is our situation, of which we are nonetheless deserving. We are only held responsible for our personal sins. But original sin, of a different nature than other sin, is a deprivation of something we are not entitled to, and in which fallen nature we are expected to follow the dictates of our conscience as guided by God, to seek restoration in Christ. It's called sin because it offends God, as He wants us to be fappy and fulfilled, and because it obstructs us somewhat from pleasing Him,and because it is properly called evil: an absence of some good.
Wisdom 2:23-24
For God created man [to be] incorruptible, and to the image of his own likeness he made him. But by the envy of the devil, death came into the world: And they follow him that are of his side.
Death and original sin are in the world regardless of whether we side with the devil/Adam in disobeying God. But we are shown that there is a choice, where we can yet follow God and recieve another fate than his.
TL;DR
We are not Adam. We are not Eve. We are not owed a nature the same as, or even similar to Adam's or Eve's. God's divine and ineffable justice is such that Adam's children recieve the same nature he had after the Fall. Their sin is still their own. Our sins is still our own. But Adam's original sin is of a different kind, and affects the very nature, because it was so fundamental: the first sin, the first privation and shutting-out of God from our lives, which are wholly dependant on Him.
