5 yoa son and 5yoa friend touching each other

My son and his friend, both 5 years old, were having a "playdate".  His friends mom called and said her son told her that my son and her son kissed each others penis'.  My wife and I had a discussion with our son explaining that this was inappropriate behavior and he is not allowed to touch anyone's private parts and no one is to touch his private parts.  When we asked him why they did that, his response was "I don't know".  One part of me says that they are just exploring and are curious and don't really know what they are doing or why they are doing that.  The other part of me is concerned about this behavior. What is considered "normal" curiousity and when does it become something I should be concerned about?