My wife had been studying with the JW's for a while, and I was just beginning to decide I wanted to do something about it, so I started trying to tweak the JW's nerves by asking them questions...one of them was "So, if Adam and Eve were the only people, how did they fulfill the command to "be fruitful and multiply"? This was a back and forth conversation over a period of weeks, as the JWs kept stammering about and saying "we'll get back to you"...consulting elders, most likely.
Eventually the original question expanded to include "Then why did Cain have to be marked to prevent people from killing him, there WERE no other people" and "Who was Cain's wife then?" which lead to "Did Cain have sex with Eve to beget Enoch?". The culmination of all this was when eventually, after about a month of this back-and-forth of questions and answers, the JWs triumphantly informed me they had found out the final answer to the whole problem: Cain DID have sex with Eve, back then Jehovah had not yet banned incest, the only reason to ban incest was because in later days, when the race had become "less pure due to sin", Jehovah knew it would cause genetic problems. His original design was so perfect, that it was ok for them to have incest to populate the earth, because there would be no genetic problems.