it does matter-for 2 reasons. Number one, it establishes the sin nature. If all humans descended from 2 people then that would explain why they all have a sin nature-they all inherited the sin nature from their parents (not their parents sin, but their sin nature) while, if it was a metaphor, and we all didnt come from two literal people, how did the sin nature come into play? I mean before the fall, everything according to the Bible was "good" so there was no sin before the fall-so naturally, the fall had to happen, and the only way for the fall to affect all of mankind, was for all of mankind to be descended from one person. (this is Biblical and repeated in Romans 5:12 Wherefore, as by one man sin entered into the world, and death by sin, and so death passed upon all men, for that all have sinned)
The second reason it has to do with Biblical infallibility. If we assume that it was a metaphor, then it destroys the infalibility of the Bible-lets look at scripture. Lets go to Genesis 2. (Im going to skip through a few irrelevant verses)
5 And every plant of the field before it was in the earth, and every herb of the field before it grew: for the LORD God had not caused it to rain upon the earth, and there was not a man to till the ground.
That makes it very clear, before the sixth day, there was no humans. and the passage continues to verse 7
7 And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.
Now with this verse, it says God forms man-now, if we stop here, an argument can be made that it is indeed metaphorical, in this context it could be a figurative usage, but if we don't stop here, and continue to verse 15
15 And the Lord God took the man, and put him into the garden of Eden to dress it and to keep it.
As you see, the text puts the word the before the man-in context, it means literally, the man, singular, not plural. There is no way to take it metaphorically here.
Now lets jump ahead to 18-20
8 And the Lord God said, It is not good that the man should be alone; I will make him an help meet for him.
19 And out of the ground the Lord God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof.
20 And Adam gave names to all cattle, and to the fowl of the air, and to every beast of the field; but for Adam there was not found an help meet for him.
Now, if man was a metaphor, do you not think that adam could have found another human being to mate with? seeing as there was none, God decided to make him one. There would have been no need for this, if there had been another woman on the planet.
If you continue on, you see then that God made woman, and in context it means singular.
I see nothing, in the passage, to suggest that this account is anything but literal-and it coincides with what the rest of the Bible says. To try and add in metaphorical references, I think destroys the infallibility of the Bible, as there is nothing in the context to suggest a metaphoric meaning.