Yes, well, 80% of us are female (in the United States, anyway). I think most people are aware of this on some level, so portraying a librarian as a woman is useful shorthand. If it makes you feel any better, there’s a funny strip out there called Unshelved. The main character is a male YA librarian:
Hi. My name is Jessamyn West and I'll be your librarian today. I work in rural Vermont as a library technologist and do user support at Open Library. My personal blog is at jessamyn.com. This blog's twitter feed @librarian.net. Feel free to contact me if you have questions or comments.