You're definitely right that if it had been just about the work, they wouldn't have spent the night together. Also, his little smile at the beginning of that episode definitely suggested that he was looking forward to seeing her in a way that was about more than just some vague nostalgia! But I really do think it's not as black-and-white as Elizabeth ultimately made it out to be, either. I strongly suspect that if they hadn't been required to have sex for the mission, they wouldn't have, you know?

-J