Okay, I have a bit of time now!
First off, sorry for calling you a nutter 
Secondly, 11 is very young, so yes I agree that a little age appropriate chat may be in order. They're not really "in a relationship" at that age though, are they? Or if there's any indication that they are experimenting physically at that age, then clearly you need to step in.
BUT!!!! But, but, but!!!! The way you think of your little boy as 'yours' and that any female who comes along is 'tempting him away from you', is a recipe for disaster.
Here goes my story.....
My sister little boy had a girlfriend. Now, granted he was 16, but she was his first boyfriend, and my sister reacted exactly the same as you. She hated the idea of her little boy being close to someone else. She hated that he wasn't 'hers' anymore.
As a result, she was cold and unfriendly to this perfectly lovely 16 year old school girl. The poor kid was left wondering what on earth she had done wrong.
The 2 kids stayed together, and their relationship with my sister just became gradually more & more strained.
They're 38 now. My sister sobbed through their wedding 10 years ago, and she has 2 grand-children that don't even know her
.
I watched for years as this poor girl pussy-footed around my sister trying to win her round. There was never anything not to like about her. She was a polite and well-educated kid. My silly sister just could not see through the fog of "my little boy!! My little boy!!"
Hope this helps a bit. I know this all seems a long way off to you, but you know, the years tick by and it all happens gradually, and before you know it, this is the result.
x