The man has no shame! Men, women, aliens of any species, it makes no difference as far as the Doctor can tell. Captain Harkness will flirt with anyone. Given the slightest hint of encouragement, he won’t stop at just flirting either. Unbelievable! He can’t be trusted with an innocent greeting; he says ‘Hello’ like he’s inviting whoever he’s addressing to join him in bed!
The Doctor isn’t a prude, or perhaps he is, but the fact remains that he can’t take Jack anywhere without putting interstellar relations in jeopardy. He’ll have to keep his new passenger on a tight leash.