I know people will have different opinions on this. But I tend to believe they could learn to know the difference.
We allow our dogs on our bed. However, a few years ago, Zoee was skunked. Well, she wasn't allowed on the bed for a couple months. She'd jump up there, but I'd tell her to get down. Eventually she would sit and look at me waiting for me to invite her up. Both of our dogs are very good at the down (or off) command. I actually only have to snap and point and Zoee understands what I mean.
I'm actually surprised to hear that the dogs haven't snuck up onto the bed after you are asleep.
I know some people (and it may differ, since every dog is different) believe it is very confusing for the dog. My brother has a rule in his house that the dogs do not get on the furniture unless they are invited. They do very well with that. Of course they sit and give you the puppy-dog eyes. LOL