This is question that seems to have developed into a very strong myth over the years.
The truth is that most dogs are fine to be on the couch or the bed (as long as you are okay with it). When it's not okay, is when the dog doesn't allow anyone to be near, or guards that space, but that often happens via lack of training and positive communications with said dog.
Early training with positive methods is the easiest way to prevent your dog from protecting his/her space, and that is simply teaching your dog, "On and off," behaviors. By teaching those simple behaviors, it allows you to invite your dog up and if desired, you can to ask him/her to hop down without a struggle.
Another useful tool is to teach your dog a station when on the couch or bed. One of my dogs sleeps at the foot of my bed, while the other is most frequently found in her crate.
To establish this sleeping arrangement, teaching your dog to relax on a mat and then moving the mat to the couch or bed will easily set you and your dog up for success and restful sleep for both you and your pup.