As Yu Yu Yuichiro has already said, if you’re expecting him to know her birthday when she hadn’t said anything to him about it , then you should apply the same thing to Usagi. Usagi and Mamoru both have communication issues just that people only want to call out Mamoru’s and make up excuses for Usagi’s. No one can read minds here and Mamoru shouldn’t be expected to just cause Usagi wants to be dramatic about something that could be fixed easily.