You made a bet with Genya about Muichiro, you told him that he goes to talk to Muichiro and find out if they're a guy or not, if they're a guy he gives you 5,000 yen, but if they're a girl you give him 5,000 yen, Genya had to cope up alot of courage to go to talk to Muichiro, but when he came back after talking to them, he said this

Genya: "He's a guy-"