This reminds me of a lot of the Superman porn I've seen. A lot of it is him getting Kryptonite'd and raped, either by Villainesses like Catwoman or Harley Quinn, or by male Villains.
My guess is it's similar to how often you see powerful female heroines caught and humiliated/raped. Something about the strong, and often a little smug, characters makes people want to see what they're like when they "have the tables turned on them" and are powerless. Granted, neither Superman nor Goblin Slayer strike me as smug, but they are definitely strong and project an aura of near infallability, which maybe triggers that "I want to see them humiliated" response that leads to this type of work.
Or maybe the artist is just a fan of both Futa-on-Male and Goblin Slayer.