'No' is normal
People in all kinds of relationships say no to one another all the time.
Bosses say no. Employees say no. Spouses say no. Friends say no. Exceptionally close friends say no. Girlfriends say no. Boyfriends say no.
Everyone who regularly spends time with someone will also routinely say no.
Some people will want you to treat no as a special word to be used only in emergencies. Those people are wrong.
Saying no is normal and routine.