It could be the cats attempt to be 'dominant' and claim it as part of his territory. Try moving the dog bed into a room the cat doesn't go into, or getting something that could scare him away from the bed. There are these things (I don't remember what they're called, sorry), that squirt air when they sense movement, it could keep him away, unfortunately, it may also keep the dog away.
This is good if the dog only uses it to sleep however, as you could just remove it when the dog goes to bed.
Change the bedding. Cat's have highly advanced senses and the cat smells the urine even after you wash it, so he/she is continuing to claim the bed. The cat and dog are probably working out some dominance issue and the cat is feeling territorial.
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