Interesting distinctions, Sunstone! I'm also curious about the purpose of making these distinctions. It seems to me that you're setting up an opposition in order to condemn certain things.
Pornography = bad.
Erotica & Art = ok/good.
With these distinctions in place a person could write ethical theory and legal code.
It is not my intention to condemn anything, Rob. You're putting words in my mouth that aren't there.
...I generally define pornography as material designed to arouse sexual feelings in the viewer and containing explicit depictions of sex.
Do you distinguish between pornography and erotica?
So we might go back to an assumption of the question. Why do you think certain forms of sexually oriented material are degrading to the model?
If we abstract one trait or characteristic of a person from the whole person and then say that one trait or characteristic of them is all that matters about them then we have, almost by definition, degraded them.
That is not just true of porn. It's true of racism, sexism, ageism, and any other degradation of people that is based on the logic of abstracting one trait or characteristic from the whole and then asserting that one trait or characteristic is all that matters about them.
I define porn as that which reduces a person to their sexuality and nothing more than their sexuality. In principle, that's no different from reducing someone to their ethnicity and nothing more than their ethnicity. Or reducing someone to their gender and nothing more than their gender. Or reducing someone to their race and nothing more than their race. Any such reduction of a person is degrading a person.