I asked Karen to work for me as a porn actress. She asked for the lead in one of my mainstream movies to do so. I gave her that. Still, she doesn't accept a porn position! Surely this is like Rihanna's, not a bug but some plot I'm missing. What is it?

I presume that this is actual content that has yet to be added. The porn studio is a more recent addition to the game and requires a lot more work. Making porn is not really worth it in the current version - you just get a short generic paragraph for the plot of each release and the business is only profitable if you cast your slaves.