Recently I was asked whether or not it was okay to allow your dog to sleep in the same bed with you. To give you some indication of my stance on the issue, both of our dogs spend a lot of time on our bed. Sometimes we'll have both 65 pound dogs and 2-3 cats lying around our bodies on a queen-sized bed. The dogs don't stay there all night because I tend to kick with my feet if they lay on top of them, something they quickly learned.
So why would you not want to share your bed with the dogs? There actually are a couple of good reasons.
Some parasites can be transmissible to humans, particularly roundworms and hookworms. The eggs from these parasites have been found on the fur around the rectum, tail, and hind legs, not just in the feces. If a person comes into contact with these areas on an infected dog and then touch their faces or food before washing hands there would be a risk of swallowing the eggs and becoming infected. Somtimes the worm eggs can actually be regurgitated into the mouth and mix with saliva, leading to infection risk when being licked around the face. If a dog sleeps with you these parasite eggs can become attached to the bedding and become a source of transmission.
Now that I've grossed you out and made you push Fido off the bed, let me try and set your mind at ease. A dog properly cared for should have minimal or no risk of having these parasites. All heartworm preventatives on the market also prevent hookworms, and most of them will cover roundworms as well. So if your dog is on consistent heartworm prevention your risk is minimal. It is also recommend to have a fecal sample checked 1-2 times annually to screen for these parasites. Obviously I don't worry about it in my family.
The other reason has to do with behavior. In a dog's mind the human furniture is a place of authority and power. Those who sit on it are "higher" in the social hierarchy. This really isn't a problem for your average dog as they are comfortable with their position in the "pack". But sometimes dogs will become aggressive towards the humans or their canine housemates. In these situations one of the first things that should be done is to keep the dog off of all furniture. This includes the bed! By restricting them to the floor and off any human objects we are working on their psychology and forcing them to a lower rank within the family. If a dog is used to sleeping on the bed this can be a difficult transition for them and the people.
With precautions and care, I don't think there are problems with sharing your bed with your four-legged family members.