I feel like if someone asks a writer to write them something that hits a kink of theirs, that's a sexual interaction.

This really rings true to me. I definitely know that "omg write me [something] because that is SO HOT unf unf unf" makes me feel the same way that being asked to take off my shirt or whatever by an obviously lascivious person. They're asking you to perform an act that is all about their sexuality. Which isn't bad, not in either case--but it's not sexually neutral, either, and both circumstances make me uncomfortable if they're unwanted.