Manners

Opening the door for a female is not a way of hitting on her... So if you do it for that matter stop doing it at all... She can open that door herself. 
Saying "thank you"  after someone opened that door for you ladies is not answering to the malicious intentions,  so if you say it for that,  just don't. 
Let's just be civilized,  and not people with hidden intentions... Or at least don't hid  it behind manners. That only shows cheapness and lack of character.