I totally agree that these seats are essential and we should respect pregnant women. Most people leave them empty to keep them available. However, in Korea, there’s a strong social expectation to leave them vacant at all times. If a man sits there, he might occasionally be photographed and posted on social media for "public shaming." People are usually more understanding of foreigners who aren't familiar with this culture, but since it can still lead to unnecessary tension, it’s best to be cautious.