Question for Christian parents about teenagers/dating/?

I think you are right when punishing your son - he is your child, and you can do whatever you think is necessary. But, I think that it wasn't the correct decision to talk to the girl about your beliefs; you should have just cut it down to a short 'if your parents saw you now...' comment - not about God or anything. Your son should not have been engaged in that activity - this is the age where kids will be doing that sort of thing, and that's when you need to do what you already did to put a stop to it.