How to talk to adopted kids about the birds and the bees

Talking to adopted children about the birds and the bees can be more complicated than talking to biological children. Their stories are more complicated and a simple "When a mommy and daddy love each other very much they have a baby" doesn't necessarily work. If the birth parents were not together, or the child was conceived out of an act of violence, that explanation might do more harm than good. How do you talk to your children about sex?