Don't fool yourself! JWs aren't any more faithful sexually, than members from any other bible-believing church. The JWs are just a little better at hiding their naughty behavior due to the penalty of shunning; and due to the policy of secrecy surrounding judicial committees.
An elder friend once said to me 'if you knew what went on that never becomes congregation knowledge you would be amazed'.
And that's just the stuff that comes to the attention of the elders.