I suspect that many partners are into the cuckold scene, and they vary from wanting to watch, or simply either being there, or knowing that their other half is getting a good seeing too. Besides all the comlications that the Cuckold brain has, it's really not a lot different from some of the kinks that those who look for more specialised services. But they can get their kicks without having to pay!
I have been made aware of this situation by one of my regulars, and should I wish to let the partner watch. Then the rate would be reconsidered. Not an option I've tried as yet, but in reality not a lot different to a party, or the swinging scene.