Kathy Tavormina, there is a picture in this book, that is said to be suitable for age 4 and up, though I did not see it till my dd was 7 and even then was a little nervous, but that may just be me:
It's Not the Stork!: A Book About Girls, Boys, Babies, Bodies, Families and Friends.
Also, have you seen this?
Wits - Julia Sweeney "Sex Ed." Monologue -...