The fall of Adam was not about sex since sex was part of the original plan, i.e. God blessed them and said to them, "Be fruitful and increase in number; (Ge.1:28)
It seems to me that the tree was figurative rather than real because the Bible is too sensible a book to come up with the fairytale idea of a tree whose fruit gives you knowledge. If the tree is figurative then the nature of the sin is not important, otherwise the Bible would tell us what the sin was. In which case it is the fact of disobedience that is the emphasis not its particulars.
This being the case, we come to what came about as a consequence of the fall and why? It is interesting that before the fall:
The man and his wife were both naked, and they felt no shame (Ge.2:25)
But after the fall:
The eyes of both of them were opened, and they realised that they were naked (Ge.3:7)
In both verses they were naked but in the first they were unaware of their nakedness and in the second they were all too aware of it. What occurred to effect this change?
It seems to me that up to this point their regard was entirely to God. They were unaware of their nakedness because they were unaware of themselves, that is to say they were unselfconscious. They could walk around naked unselfconsciously, they could have sex unselfconsciously because God was the centre of their world as they walked with him in the cool of the evening.
A single act of disobedience brought a devastating sense of themselves. They dethroned God and became the centre of their own world, becoming aware of themselves and thoroughly self-regarding for the first time. This is why sinfulness is in the nature of mankind and not simply an option we choose. Self-consciousness is much like becoming aware of anything for the first time. Once you realise a thing you simply must live in light of it, or go against your own nature in denying it. This is why such an incredible solution was necessary to put it right. It is in coming to faith in Christ that our attention and focus moves away from us and back to God in Christ. This is a miracle.