If you can't say no to someone without facing a negative reaction (such as pouting, begging, withdrawing, attacking, or response-blaming), they are not giving you the option of true consent. If someone can't say no to you without worrying about how they will deal with your reaction, you are not giving them the option of true consent. Coercion is the sneaky underpinning to this -- sometimes unintentional, but every bit as much a problem whether it is intentional or not. It doesn't become harmless just because someone isn't doing it on purpose.
Expressing disappointment is fine -- as long as you're not making the other person feel like they are responsible for making you feel better. Usually you will have to overtly take responsibility for handling your disappointment for this to work. For example: "I'm feeling disappointed that you don't want to [do the thing] with me. I'm gonna go [practice self care] to feel better; I'll be back" <- that is great as long as it isn't passive aggressive but is sincere effort to handle one's own emotions.
What is not okay is "I'm disappointed that you won't [do the thing] with me." *waits for the other person to make me feel better* or "I'm disappointed" *goes in another room to sulk and wait for them to come make me feel better*
That is effectively punishing them for saying no by making them do emotional work in order to have a positive environment. If you can't process out of your disappointment very very quickly, don't do it in their presence.