Yes!! Animal Talk is amazing...I got Penelope Smith's book for Christmas. If you follow her instructions I am sure you will be able to talk to him yourself. It is really quite simple, which is the hard part
I got the idea that it isn't Desi and Duma that he is trying to keep away...he thinks that if he doesn't mark you, stray cats are going to be coming to you, and you're house will be over run with cats. He said he goes outside to make sure the other cats know that you're house is HIS house, and they aren't welcome there. I told him that peeing on you isn't keeping the cats away, that you are doing it. He actually seemed relieved that he didn't need to go outside anymore, he said it was a lot of work. He feels like it is his job to make sure you don't get over run with cats. Even if you don't say it often, it could have been just once where you thought/said you were a crazy cat lady, and maybe pictured yourself with many cats.
I told him if he wants to protect you, he can do things like catch mice and bugs. That you would appreciate him doing that more than keeping the other cats away.
Maybe you could tell him that you aren't planning on getting any more cats? Let him know that you have it under control, but thank him for wanting to help you. Encourage him in other way he can be useful. I told him that snuggling is a great way to protect, as if anything were to happen he would be right there to wake you up.