You don't have to laugh at "girl math" or "girl dinner" or "I'm just a girl" jokes. You don't have to laugh at misogynistic humor that demeans women. You don't have to go along with it. You don't even have to sit there awkwardly pretending you didn't hear anything. You are allowed to speak up and say something.
You don't have to be a jerk about it. The most effective yet civil way of dealing with harmful "humor" is just pretending you don't get the joke and asking them to explain. "I don't get it, what's girl math? What's a girl dinner? What do you mean by you're just a girl? Can you explain the joke to me?"
And if you're feeling bold, you can just say (or follow up with) "I think that's misogynistic". That's allowed. You don't have to quietly endure misogynistic "jokes", you can call a spade a spade.
