There's actually an entire podcast on "Savage Love" dedicated to the subject of whether "douchebag" is offensive to women. Many women called in and said that since douching has been discovered to be harmful to the vagina, it's good that douchebag is used as an insult, because it reinforces that douching is a bad thing, hopefully discouraging women from partaking in it.

Doesn't mean that people won't be offended by it still, but an interesting way to think about it.

Really interesting. I could see an argument being made there. I think the bigger thing is that douche is relatively disconnected with women. It's not as direct as, for instance, calling someone a "pussy", which is more directly negative toward females.