You might want to go to the library and look up the answer together. They offer a chance to teach your children what you know or believe about sexuality. You can even make use of a TV program that you believe sends the wrong message. I want to tell you what I believe and why. You don't need to wait until they ask a question.
Many children never ask questions.
When our children were young, we didn't wait until they asked about whether they should look both ways before crossing a busy street, or whether they could touch a hot stove. Some things are essential for them to know. Learning about sexuality is the same. You need to decide what is important for children to know, and then tell them before a crisis arrives. A good time to start might be after a TV show or news broadcast that deals with a sexual topic.