If my pet is approached by another dominant for anything besides the most casual of conversations, she is required to ask permission to even speak to him (or to continue the conversation, if it was already going). Often, that permission will be denied.
I don't feel any more insecure about this than I feel insecure for locking my front door at night. She's my property, it's not her decision who she can and cannot talk to. It's mine, and she trusts me to make it intelligently on her behalf.
When a man comes into the periphery of her life, I want her to be constantly on edge about how I will react, and conscious of my approval with regard to her interactions with him. Hell, if she flirts too much in a DREAM, she'd better wake up and apologize.
I'm going to offer the minority opinion (for this thread, anyway) that a Master often does know best in this regard- if I as a Dom issue an edict preventing communication with Doms besides myself, I'm making a very symbolic statement and risking little. Let's face it- there are a lot of mouth-breathers out there who are not worth talking to. I think my pet can survive without hearing PrinceVegetaDom7626's opinion on enemas.
And of course, if she doesn't like the way I curtail her, my pet can always walk away.
She's still right here, clinging to my leg with her eyes lowered to the floor. That should tell you all you need to know.
-IC