Sometimes people with low self esteem find themselves getting ashamed about the most innocuous stuff and trying to hide it from people they care about the opinion of, even when they know there's no reason to be ashamed. Then once the instinctive mistake is made, they can't work up the courage to correct it before it becomes an actual problem.
You've got an insecure and inexperienced MMC getting blindsided by an on-the-nose question from his equally insecure FMC not-girlfriend that he's desperate to avoid conflict with, about a confusing meeting with the FMC's coworker/friend. If he literally tells it like is, he's potentially creating a blow-up/fallout between them, despite the fact he's pretty much already dealt with the situation by telling the coworker he's not interested. So I think it's pretty realistic that someone in that situation might instinctively take the path of least resistance and know it's a mistake.