onacruse,
Just from reading that account from the Bible and only the Bible, the original sin did not have anything to do with their sexual organs. As stated by the Bible in Genesis the 3rd chapter the original sin was an act of disobedience. Eating from that one specific tree was off limits.
So, if the "sin" of Adam and Eve was eating a forbidden fruit, then why wasn't their first impulse to hide their hands? or to wipe off their mouths?
That question is besides the factual point. There is no "IF" when it comes to the original sin. It is very clear within that account that it "WAS" the sin. We cannot be sure why they didn't hide their hands or wipe off their mouths, unless you mean that in the sense that God "caught" them in the act, like someone would catch a little kids hand in a cookie jar and he quickly pulls it out and hides his hands behind his back. If that is how you mean it, which is how you are putting it,
"Now, ordinarily, when we get caught doing something "wrong," we try to hide the "offending" member (the child tries to hide his hand that has the 'stolen' cookie; we hurriedly try to pull our pants back on when Mom walks in unexpectedly while we're masturbating, etc.).
then there is an answer for it. The Bible says that God did not catch them in the act, it wasn't until later on after the act, when God confronted them about it. So there is your answer! It is quit easily seen when you read it.