Gen 2:24-25 (KJV) states "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
And they were both naked, the man and his wife, and were not ashamed."
This indicates to me that Adam and Eve were married (i.e. the use of the word 'wife') and were having sex (i.e. 'and shall be one flesh.'). This passage is written before the original sin takes place which results in them knowing that they were naked (i.e. 'And they were both naked, the man and his wife, and were not ashamed.').
Therefore I do not believe that the original sin was sex.