"My 8 year old son is too sexually aware" -Worried Mom

I really need some advice. My 8 year old boy is too sexually aware. In the last few months I've noticed him touching himself, rubbing himself against anything he can find, and doing things that I'm too ashamed to type here even though I am anonymous. He is touching his younger sister in ways that are not brotherly. He's fixated with breasts and grabs any he sees. He also forcefully tries to kiss people, both male and female. I've smacked him several times but it hasn't stopped the behaviour. Just last week, his teacher told me he was caught kissing a classmate. I understand children are curious but I have no idea where he learnt all these. He has never seen that kind of behaviour in my house. I monitor what he watches on TV and I have never allowed him on the internet. My husband thinks he should be punished for his behaviour but what kind of punishment? Should I lock him up in his room till he's grown? I am a mother who is so ashamed of her own child. I'm afraid to take him out so he doesn't start harassing other people's children. I'm so confused and ashamed that its making me sick. Can you please help me make sense of this? What do I do to stop this behaviour?