Sex was indeed just sex and could remain just sex for quite a while, but both parties had to want it to stay that way. Cora wasn't a telepath, so she couldn't be sure that William didn't want to change it. Now, though, she was, so the topic could be dropped. Their purely sexual relationship would remain purely sexual, despite this date thing or whatever this outing was. Most likely, it was just an exception to their rule and they'd be back to tearing up his apartment the next time and the time after that.
What lie she would tell was indeed a good question. Just finding them was out of the option. Maybe a Goodwill trip resulted in an unexpected catch? Really, Cora had no idea at the moment. She'd probably conceal the earrings in her room until she thought of something good enough. "Don't know yet, but I'll think of something." She had to think of something she hadn't used before, too. Concealing everything about their romps was fun and the risk of discovery was kind of hot, but keeping track of her stories was somewhat difficult.
And of course, Cora's brain just happened to think of a weird suitable movie reference, to another moment that involved jewelry and nudity. "So are you going to fuck me like one of your French girls?" she asked, smirking seductively at William.