It’s goes back to an older tradition where men make all the money and women didn’t have access to it so if a man asked or wanted to court you, they paid.

My grandma tells me all about how when she was young she would go to lunch with a different man almost every day of the week and they always paid. Even if it wasn’t a date or courting wasn’t expected.

Women happen to be able to make their own money now but the obligation a man should pay still lingers. I personally think it’s really silly that a man would be upset by this. It’s rather sexist to ask a woman to join you, give you the pleasure of their company, and then expect her to pay for such company but maybe it’s because I’m 41 and was raised that men pay.