When Buffy exposed her midsection it was obvious that she didn't have the body of an incredible athlete; not of a martial artist or a gymnast or even a marathon runner. She had a thin, but otherwise other unremarkable looking body when it came to muscle tone. No visible six pack. The strength? The coordination? Completely supernatural.
Once John finished she straightened her shirt.
"Sure thing!" Given how casually she accepted his offer, if there was supposed to be anything more to his dinner invitation the slayer completely missed the context. They were dinner buddies. Movie watching buddies. Help each other get uncursed buddies.
She waited for him to leave, changed into his shirt and joined him downstairs.