Looking at porn and looking at ads for actual sex are two very different activities. Looking at porn is just about fantasies and masturbation. Looking at sex ads is window shopping that could lead to cheating. I don't see anything wrong with your guy looking at porn, as long as it isn't the nasty illegal stuff, but looking at sex ads sounds like he is thinking about cheating on you. Pick your battles, so tell him that porn is okay, but looking at and exchanging emails involving casual encounters or prostitution is not okay with you.
Good decisions come from experience. Experience comes from bad decisions.