My daughter is 16 and she has watched films with us all her life. When she got to 12, we watched certificate-12 films with her. When she got to 15, we watched certificate-15 films with her. Inevitably, the 15 rated films had sex scenes in so we watched those together and we discussed anything she wanted to discuss.
Maybe that is a good way ahead? It's very uncomfortable watching sex scenes with your child, but if you can try to push any embarrassment aside, it will hopefully make the child realise that sex isn't anything to be embarrassed about and it's a normal part of life.
I hope that helps 
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