I don't like my girlfriend's fake pregnancy april fools joke
Why did he joke upset you so much?
Because I think it was an immature thing to do.
I think you need to remember it was only a joke.  There's no need to get so upset over it since they weren't being serious.  Just forgive and move on!
Yeah I guess so, but I don't like people doing that kind of shit.
I understand that you don't like, and maybe you should tell her that. But for now i'd just take it as a harmless joke and move on.