Because only Spartan women give birth to real men.
5 seconds sound clip from the 300 movie soundboard.
You can hear this line at 00:09:42 in the Blu-ray version of the movie.
Quote context
[...]
- My king and queen, I was just entertaining your guests.
- You rode all the way from Persia for earth and water?
- Do not be coy or stupid, Persian. You can afford neither in Sparta.
- What makes this woman think she can speak among men?
- Because only Spartan women give birth to real men.
- Let us walk to cool our tongues.
- If you value your lives over your complete annihilation, listen carefully, Leonidas.
- Xerxes conquers and controls everything he rests his eyes upon.
- He leads an army so massive, it shakes the ground with its march. So vast, it drinks the rivers dry.
[...]