I also have always felt that I wanted to be the person who talked to my kids about sex first, rather than having them learn about it from someone else.
And then when my youngest dd was, maybe 3?... her older sister, who was maybe 5?... asked what a vagina was at the dinner table... and younger dd answered! She knew because another child at preschool had told her. So I had a bit of a jump-start with talking to my kids lol!
Oops, sorry... that was us
DW's mom was a nurse midwife for ~20 years. Young kids ask about this stuff... there are good books for that age group. You present it factually and then they just move on... sort of like farm kids in the old days. Without the hormonal overlay, it is so much easier to present it factually at a younger age. You keep it very vague unless they really push you, but you do give an honest account. It isn't icky or sexually loaded... it's just another curious fact kids have learned.
Then when you present it in adolescence you can focus on the moral/practical issues with out all the tittering and guilt about the mechanics.
PS. DW's mom is also a strident evangelical... so this isn't a religious issue per se. The moral issues above could change that equation but you don't have to equate sex ed and family planning(though you probably should).
