How is it possible, from a rational point of view, to believe that Mankind merit damnation, because of Adam's Fall?

Two explanations are possible here, or a combination thereof. First is what is known as the "Federal Headship" of Adam.

Theologian Millard J. Erickson explains the concept, "A reference to the view that when Adam sinned, he was acting as the representative of the human race; consequently, the whole race suffers the consequence of Adam’s first sin" (Concise Dictionary of Christian Theology, p.56).

So Adam represented us in much the same way a representative represents his constituents in a representative democracy. Or, to put it another way, Adam did what we would have done if we were there.

The second explanation concerns original sin.

John Calvin explains:
... the Lord entrusted to Adam those gifts which he willed to be conferred upon human nature. Hence Adam, when he lost the gifts received, lost them not only for himself but for us all....

There is nothing absurd, then, in supposing that, when Adam was despoiled, human nature was left naked and destitute, or that when he was infected with sin, contagion crept into human nature....

Hence, as Augustine says, whether a man is a guilty unbeliever or an innocent believer, he begets not innocent but guilty children, for he begets them from a corrupted nature....

Original sin, therefore, seems to be a hereditary depravity and corruption of our nature, diffused into all parts of the soul, which first makes us liable to God's wrath, then also brings forth in us those works which Scripture calls "works of the flesh" [Gal 5:19]. And that is what properly Paul calls sin" (Institutes of the Christian Religion. Vol. 1. Philadelphia: Westminster Press, 1960, pp. 250,251).

Scripture references on the above points would be: Gen 3:1-20; 6:5; 8:21; Ps 51:5; 58:3; Jer 17:9; Rom 5:12-21; 1John 3:4.

So we are condemned before God because we inherit a sinful nature. And this sinful nature inevitable leads us to sin. Or, to put it simply, we sin because we are sinners (Rom 3:23).