If he is masturbating to these sites, it could be simply that they serve as pornography for him. In other words, maybe he knows that porn sites are "fake" and is only sexually stimulated by the same genuineness of the personal ads that is making you feel threatened. So in that sense, if pornography is no big deal for you, then perhaps you could just consider this his pr0n.
My concern is with his disrespectful attitude towards the pain that it causes you. Instead of talking it out, he is starting to hide it, which will generate resentment in him and pain and frustration in you. Even worse is his "I'll never cheat, and that's all that matters" response to your sexual needs. He is satisfying himself and leaving you wanting and frustrated. Never mind *how* he is satisfying himself; his categorical refusal to attend to you makes him a turd.
Maybe you could try to solve both problems by posting a personal ad yourself and doing some role-playing. Let him answer the ad: Go into opposite rooms and let him IM chat with you and call you the way he's fantasizing about doing with these strangers. Turn him on over IM and the phone... and then ring the doorbell.
[SIGPIC][/SIGPIC]Moderation in all things, including moderation.