Disclaimer: My knowledge on religious matters is not very deep and originates from historical novels and some basic facts told at school. So, please take the following interpretation with the grain of salt.
As far as I can remember the reason why original sin and sex are connected is the following. When Eve offers Adam the apple, they both gain knowledge and recognise that they are both naked (compare 7th verse on http://bibleontheweb.com/bible/ASV/Genesis-3) Hence, a logical conclusion is that they make love to each other and are banished from paradise for this wicked deed (I could imagine that a hint for this interpretation is that before this verse Eve is referred to as a "woman" and after that she is refereed to as a "wife".) Therefore the original sin is the first time Adam and Eve have sex.
