Cosmo, you sexist piece of shit.
Oh boo. There’s Wanda, his wife, grinning evilly in the background as he’s saying it.
Besides, context is important. Remember what this episode was? "Queen for a Day." The one with Trixie Tang’s birthday?
The entire point of Wanda hitting Timmy with this wish was to teach a moral to him. Timmy was hitting a mental roadblock trying to come up with a birthday present for Trixie, not having a clue what “a girl” would want for her birthday. Cue the wish forcing him into a female body and… Still pretty much liking the exact same stuff. And as he later found out, Trixie liked the same things he did, even if gender norms didn’t fit for it all.
In the end, Timmy ends up giving Trixie the present she wanted in a comic book store, but fakes getting upset over it when he gives it to her because her friends thought it was only for boys. But, she WAS grateful.
And this moment? This right here? Our male protagonist, Timmy, is now getting playfully mocked by Cosmo for suddenly being a girl. That’s going to teach boys, through the eyes of Timmy, how stupid it is to be demeaning to females for being female.
This episode was basically a giant slap in the face to the idea of gender norms. And you’re still calling Cosmo sexist for reinforcing the moral, which is how stupid even casual sexism is?
Childhood is firmly not ruined, thank you.