Boy book?

I was doing my usual spreading the gospel of Carl last night at a gathering and a woman whom I had convinced to start the series popped in and said she's done the first half of book 1 and it's a "boys book".

As I am a boy who enjoyed the book and doesn't really conform to gender norms I was really surprised to hear that and wanted to know if I am way off base here thinking it is accessible to all genders? Is it the gore aspect that would throw people off? I always felt there are a lot of strong females in the book.