[BortimusPrime]

Ask them how many stone age civilizations had the females go out and hunt while the males foraged for berries and gossiped about each other in the camp. If they argue that actually happened then they're idiots or liars. If they agree, then they have to admit there must have been some biological basis for why primitive humans had gender roles, otherwise those roles would be random for each society. Then ask them to prove why the characteristics that formed that primitive biological basis don't apply today in roles that favor said characteristics.