Is 8 too young to talk about sex?

Okay, my six year old daughter told me the other day that my 8 year old son laid on top of her and asked her to open her mouth while he kissed her. I dealt with the situation by talking to him immediately. I explained that it was inappropriate behavior and he started crying so hard. He felt so bad, and it is so confusing as a parent because I don’t want him to feel ashamed. He doesn’t know why he would do that to his sister, and of course he wouldn’t, he is 8. They don’t watch adult television, but have seen some kissing scenes before. I know that it is just curiosity, but should I explain sex to an 8 year old? How would I do that? I don’t want him to feel like he is a bad person or anything.