If she encountered the word "sex" in a biology textbook, she may not have been asking about the human reproductive process, but rather about sexual differentiation in general. I'd discuss it all with her in as much detail as she doesn't balk at; she has already shown interest, and there's no reason not to present such information to a child who's intellectually ready, stripped of titillation and taboos.

I identify with your worry about feeding her interests enough and in the right ways. Why not start with some books and online resources on the life sciences, with an emphasis on evolution and the human body, and science in general? Some ideas for life sciences enrichment that may appeal to your daughter:

The Cartoon History of the Universe
Child Atlas: Human Body (you should ignore the three prudish reviews, at least two of which are by the same person; see also the free study guide by Ellen McHenry to accompany the book)
The Dawn of Life
Discoveries: The Human Body
Evolution: How We and All Living Things Came to Be
The Magic of Reality
The Science Book: Everything You Need to Know About the World and How It Works
The Story of Science
Thread of Life: The Smithsonian Looks at Evolution
The Visual Dictionary of the Human Body

http://www.bbc.co.uk/schools/ks2bitesize/science/
http://www.bbc.co.uk/sn/
http://www.becominghuman.org/
http://www.biology4kids.com/
http://evolution.berkeley.edu/
http://www.hhmi.org/coolscience/
http://www.hippocampus.org/Biology
http://www.khanacademy.org/science/biology?k
http://learn.genetics.utah.edu/
http://www.neok12.com
http://www5.pbrc.hawaii.edu/microangela/index.html
http://www.pbskids.org/zoom/activities/sci/
http://science360.gov/topic/Life+Sciences/
http://www.sheppardsoftware.com/science.htm
http://www.tolweb.org/tree/phylogeny.html
http://faculty.usiouxfalls.edu/arpeterson/science%20standards.htm
http://www.visiblebody.com
http://www.watchknowlearn.org/Category.aspx?CategoryID=122
http://www.zo.utexas.edu/faculty/antisense/DownloadfilesToL.html
http://www.zygotebody.com/

Striving to increase my rate of flow, and fight forum gloopiness. sick