Public Display of Affection issues with my parents? What can I say to make them understand? Please Help. :(?

I'm 17. The guy I'm dating, he's the second boyfriend I've ever brought home. We've been dating for 2 weeks and a couple of days. I really like him so far. Like every couple we have our good and our bad days. I'll be turning 18 in 6 months, I need my parents to understand that. They seem to have issues with our forms of PDA, which is him placing his hand on my leg and rubbing my neck. That's all that's happened in front of them. When he hugged me goodbye in front of my mom, I asked him to kiss me on the cheek which he did. I don't see the problem. I'm almost 18 and I need to stretch my wings so I can fly from the nest soon. I don't know what to do. Any ideas? Thank you.