Girls are excluded from sport or activities on the basis of gender alone
This argument relies on the historically constructed idea that certain activities are inherently gendered, and uses that tradition to exclude individuals who do not conform to it. It commits the is-ought fallacy — because something has traditionally been one way does not mean it ought to continue that way, especially when a constitutional right is at stake. Assigning activities as 'for boys' or 'for girls' is a form of gender stereotyping that the law explicitly prohibits.
If someone says, 'But it's just a joke, don't take it so seriously,' you can respond: 'Even if it's said as a joke, the law recognizes symbolic violence when messages reproduce stereotypes and naturalize subordination. My right to equal participation is protected by Law 26.485.'
After you give your response, they may push back. Here is how to handle each counter-argument.