@Tlos
Dense? He was just lured into a girl's home under false pretenses and now she's sprung this question on him out of nowhere. He promised that he'd keep his and Yoriko's relationship a secret from the office, so what else was he supposed to do?
@zergqueen
Nah, that's not smart, in my opinion. Tateyama already saw that he went on a date with Yoriko. If he said he was dating someone, but not who, she would just assume it was her, and it might spread around the office.