Just tell her the truth.. She wants to know and so educate her. It is just like any other subject you have taught her about.
Have you explained about how her body is changing and what to expect with her menstrual cycle?
Then tell her about the boys and how their bodies work.. And then explain your feelings on how people that love each other, are married.. or whatever you believe then love each other and want to have a baby and so how humans do this.
Get the American Girl books that explain all of this. and read them together.
Treat it like a summer Anatomy class. It will be interesting. and matter of fact.
Once I told our daughter, I also reminded her that these are things she could ask ME or her dad about any time, but she should not share it with their friends, to let their parents talk with them about this subject. She said "Don't worry, I never want to talk about this again!".. Hee, hee.. actually the discussions continued every once in a while as things came up.
Even though I was freaking out inside, not wanting to scare her.. I just made it very matter of fact.