I think that the difference is that an expectation is like a boundary… with an expectation you are not looking to change someone else’s behavior or force a behavior out of them. You are simply setting the expectation/boundary of what you will and will not accept.
With a demand you are trying to force a behavior out of a person.
For example, I expect that my husband …
Will not use illegal drugs.
Will not cheat.
Will act responsibly financially.
Will be loving and supportive.
Will not smoke in the house.
He’s free to do whatever he wants, but if he does not meet my expectations (boundaries) I will most likely divorce him.
A demand is when I want to get something behavior out of him.
For example when I have caught him smoking in the house. I demand that he either go outside and smoke out put it out.
Demands are often what we start issuing when a person is breaking our boundaries all over the place. We use demands when we are trying to get the other person to understand that their behavior is not acceptable.
Cause like with the smoking in the house he will need to move out because of it. I am allergic to cigarette smoke and have had pneumonia in each of the last winters. I cannot be around cigarette smoke.
People tend to look negatively at "demands". But they have a place in marriage. Usually by the tim the demands start coming out there are serious problems in the marriage and the demands are given based on sheer frustration.
Not sure if that makes any sense. But I took a stab at it.