When Should You Say You Have A Boyfriend?

This is terrible, because this is me. I'm so oblivious to hit ons, or invites. 
All I think about are harmless, happy, friendly everything... now I understand where my boyfriend and friends mean when point out the 'obvious' stuff.
It's not my fault, I would be more than happy if some guy/ new friend were to ask if I was single, I'm proud to say 'Nope! I'm taken & happy!' 
So it my defense, I wouldn't want to just assume and blare out 'Yo! Calm that dagger in your pants and don't get any ideas mister! I'm not here for games; I am here for friendship; comprende sir?"