Do porn and other remotely perceived entertainments work because of Mirror Neurons?
I think it is likely the case. I agree that the idea of a "mirror neuron" is a bit dubious, as it implies that the function of those neurons are related to mirroring. It's probably the case that they represent the actual movement, and their activation in witnessing the action is tangential to that. 

As for your question, research was done on this exact topic, with positive results. Mouras et al. found that the magnitude of erection achieved by participants watching erotic videos correlated to the activation of regions known to contain "mirror neurons". 

References
http://www.sciencedirect.com/science/article/pii/S1053811908006897