No, he's behaving normally. This is normal. Peter is a very normal guy.

"Really?" And congratulations, she just sidetracked Peter from his jealous fussing with the mental image of Lucy fooling around with a woman. Or women, multiple. He realizes he probably ought to feel as jealous over that as the thought of her with a man, but finds that he really, really doesn't. That in mind, he decides to just go with her question, because damn, that might actually appeal. "Is that an option?" he asks.