It's either admit the truth or walk away, and Peter doesn't fancy his chances keeping things friendly with Lucy if he both tricks her and then shuts her out in one night. Anyway, maybe he can still choose how much of the truth to share, rather than just let her assume the worst of him (as he's sure she would).

"I didn't want to change my mind," he says. Then, finally noticing why she stopped, he peers at her foot. "...What's wrong?"