Kaylee is a grown woman, just as he's a grown man. They made no promises to each other, and she's free to see anyone she likes. That doesn't mean when Simon finds out, he won't be taken aback.

"Okay, all right." This is mostly humoring River, but all the same, Simon's aware that her gift makes this something more than the influence of a month and a half-- and someone is going to catch his anger about that, later-- without her medicine. "Where do we find these sticks?"