So to avoid the classist implications, would it be possible to have a character be and stay middle or upper class while still descending into being a slob? Or to have an educated white-collar man become a slob without losing his job or intelligence?

He sure could, but if you’re honestly going to try and write inoffensive porn, it’s not going to be very exciting. Write what you want to write, but at the very least be aware of what you are writing. My redneck stories rely on stereotypes and classist tropes–I’m up front about that. To some people, they would most certainly be insulting. But porn is, in many ways, designed to insult. Just be aware of who you’re insulting, and do it for good reasons.