Many of us don't have all the education we should have when it comes to anatomy; school isn't always great at teaching the basics, and it's left a lot of people walking around with some pretty strange ideas about how genitals work.
However, it's rare that an adult thinks in such a way as u/throwingawaylateeer's former date. After clarifying while they were hanging out that she was on her period, he revealed that his knowledge of that time of the month was severely lacking. She wanted to know if she was in the wrong for laughing at him, but honestly, who can blame her?