Why is consent such a difficult concept?

I was talking to one my my male friends the other day and he was telling me about some news article he read.  It was a story about how in another country...India maybe...there is a new method of deterring rapes.  

Basically women are given this contraption that fits inside their lady parts.  It contains inverted razor blades which are no harm to the woman BUT if a man sticks his d**k in it he will regret it.  It will go in with no issues but when pulling back the "barbs" dig into the skin and the contraption slides easily out of the woman.  The man has no choice but to go to the hospital because the only way to remove it is surgically.  

My response was GOOD! Maybe that'll deter would be rapists...after a few men end up in the ER.  My friend said "but...but that's so archaic.  It's based on a medieval torture device!" I was trying to be patient so I said "I'll never understand why men think they can stick their d**k in whatever they want! It is not your privilege to f**k anything that moves." My friend finally said "I've never really thought of it that way..." 

My point is my friend is a good guy.  He wouldn't force himself on anyone...he'd KILL anyone that touched me or his sisters.   But he still couldn't grasp WHY any woman would need to go to such drastic measures. Why aren't men....boys taught they can't just put their d**k in it?!? I don't understand!!!