Because he doesn't want to? If his was a girl saying "later", the guy would be seen as an asshole for being angry that she won't have sex with him when he wants.
She's not angry, though. If she said it made her really angry, I'd call her out on it. Getting mad at people for not having sex with you isn't ok, no matter what gender you are, but that's not what's happening here. She just said she was frustrated and confused.
If his was a girl saying "later", the guy would be seen as an asshole for being angry that she won't have sex with him when he wants.