I agree with Don's response and Pat and AJ and I usually say something similar (which also happens to be the truth.)

We do have models ask all the time if we (production staff) are getting turned on by what they're doing, but we generally reply that our focus is on creating a fantastic product that will turn on a LOT of people and that requires a lot of focus, and they usually understand that. We usually also add that, when you've shot 400+ scenes, it sort of ceases to be anything other than a focus on the production aspects of the work.

I think many models derive their sense of self-worth from being attractive and arousing to everyone they meet (which, honestly, isn't very healthy) and so gently reminding them that they have (or will have) many fans and that they look great... while also keeping focus on the professional nature of the shoot... usually serves the dual purpose of keeping them aroused and focused and keeping the setting professional.