Property is theft in the same way that religion is sin

No. What he was saying is that everything you do starts off as a thought. So instead of policing your own actions, police your own thoughts and don't entertain the bad ones when they enter your mind. Then when the opportunity presents itself - let's say she indicates she's up for a quickie behind the bike sheds, if you've already been fantasising about all the different scenarios you're much more likely to take her up on it than if you haven't. Therefore you should consider thinking about adultery as being just as bad as the act itself, even though it really isn't.