You are not naive,just want to hear nice and loud.
When this guy sees you,there is a bulge in his pants because you arouse him and he gets a feeling of having sex with you there and then but
he is avoiding you as he realizes that he cant do it and doesnt want you to know ( unfortunately you have seen).
Friendship has changed,you realize it or not.He now has seen you as a woman ,not just a friend.
I hope you can understand now.