You might want to go to the library and look up the answer together. They offer a chance for you to teach your children what you know or believe about sexuality. You can even make use of a TV show 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 are young, we don't wait until they ask to teach them they should look both ways before crossing a busy street, or 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. It is okay to feel uncomfortable.