Sally is 16.

Absolutely. If you bought a used copy, the book just fell open to that scene.

I've seen a number of people comment that they can't believe that McCann is that sexist, both here and on some other sites. I don't understand that at all. Sterling Cooper was that sexist at the start of the series, and Joan and Peggy are the exceptions to the secretarial pool, not the norms, even there. The time…

Just like if for some reason the writer wanted to indicate that the occupant was a prostitute, he might have specified certain clothes, a guy hastily putting on his pants, and some money on a dresser. Is that all the forms that prostitution can take? No. It's just a quick thing.