"It is a novel written by Female Author. That means, it is the feminine fantasy and not otherwise."
Why has it been read by male?
It's because the story, depicts in the movie, is an abridge version indicating a male being sadistic, while, upon reading the novel, it's completely different and it's all about the feminine hidden fantasy and inner self about how they behave inside with the very small portion of outward behaviour apparent to the male gender.
This is the main reason, this novel has been read by male, which is to get to know or to decipher female mentality like how attentive females are, while interacting with the world on a day-to-day basis.
The protagonist being sadistic, intertwined throughout the novel, is just the part of the author's negative thought process, just like a twist in the plot or slightly a villain of the plot in this story, just like every story has both the sides: positive and negative; obviously, the author tends to showcase herself as the Hero of the Story, which is the mindset of every individual while conveying their message to anyone through any medium.
This vast difference can easily be understood after reading this novel, which is arduous as well as time-consuming task in the current fast-paced life, especially for those who are unwilling to participate in this tedious activity to sit and read novels. Still, reading is the most mature way to understand and to learn things thoroughly.