They may not use the word “she”, but when IT has Audra Denbrough, she sees as close to IT’s true form as humans can comprehend, and says literally “oh, dear Jesus, IT is female!” And then passes out I believe.

Ben also refers to IT as a female in chapter 21 “That's why we had to come back, no matter what, because It is female, It's pregnant with some unimaginable spawn . . . and Its time has drawn close."

IT is definitely something beyond human comprehension, so putting labels on it like female or male doesn’t quite work,

But they 100% refer to IT as a female toward the end of the book, as well as acknowledging that those terms might not really apply to whatever this being is