Does he explain why it is a sin?
In this case, the idea would be, “Sure, I have a bunch of things I should be doing, and I know this is just some stupid softcore S&M novel, but still.... there must be something to it, if it’s so popular. I think I’ll check it out.”
At best, it’s a waste of precious time, and the act of reading it increases one’s tolerance of vice.
Curiosity:
http://www.newadvent.org/summa/3167.htm