Your vagina self-cleans itself so douching is not required (my oven also self-cleans).
Furthermore, douching upsets the delicate balance between good and bad bacteria in the vaginal ecosystem and can actually be harmful (AKA yeast infection = sad Vagina).
So, when calling someone a douche you are accusing them of causing yeast infection.
Ive had a few women that told me that you can douche to prevent pregnancy (ranging in age from 25-38)
This does not work.
In fact: it is an epic fail as it leaves you pregnant with a yeast infection.