I think it's incredibly sexist.

I also find, though, that it has tremendous force as an insult.

(shrugs)

Do I contradict myself? I contradict myself. I am vast, I contain multitudes...

Thinking about it, though, it's not the sexist aspect that is at the root of its insultive power -- it is, as you point out, the "uselessness" or, pointlessness, of the process and, ergo, the tool. By calling someone a douchebag, the connotation, to my mind, is that they're useless.