Love could be a theme. The relationship between the princess knight and the kept man might involve romantic love or a different kind of deep affection.
If Spider-Man held onto the symbiote, it might give him an edge against villains, but at the cost of losing control and potentially becoming a threat himself. It could also cause internal struggles as he tries to balance the power and its negative effects.
I'm sorry, I haven't read this novel specifically. But generally, a 'kept man' might imply a male character who is supported financially or otherwise by the princess knight in the story. Maybe it's a story full of unique relationships and adventures within a fictional world.
If Spider - Man kept the cosmic powers in fanfiction, he would likely face new challenges. With great power comes great responsibility, and these cosmic powers might attract more dangerous enemies. He could also struggle with controlling the powers at first. His relationships with his friends and family might change as they try to adjust to this new, super - powered version of him.
It could mean don't be like a flat or unoriginal piece of pulp fiction. Pulp fiction often has a certain formulaic or mass - produced quality, and being'rectangular' might imply being rigid or one - dimensional in a similar way.
We should not tell sex stories because they are often inappropriate and can make others uncomfortable. Such stories involve private and sensitive topics that are not suitable for general discussion.