I just saw video of them at the read through, and everyone had great fun with that line. That kind of stuck with me; it also emphasizes a bit how Ten sometimes mumbled and it could lead to a misunderstanding, and how Donna definitely didn't see him as a romantic interest. All in one short scene. Sets the tone for the season, sets the refreshing difference between Donna and his other two companions as Ten. She totally viewed him as an alien and wanted nothing to do with a "relationship" with him.

You're right about Donna. Although she was very compassionate and sympathetic, she realized that they had a job to do. Best to get on with it, learn from it, and move on. More about that in "Fires of Pompeii" and "The Doctor's Daughter."