I think it's less a function of gender than age. Slayers are called young and generally die young. I think it is assumed that a slayer is under the (patriarchal and parental) care of her watcher, thus he is expected to take care of her financial support (which brings up more interesting questions about Wood's mother, aka the Subway Slayer). This is probably why Buffy was willing to accept financial support so readily from Giles when things fell apart. It is certainly a way of exerting control over a slayer.

I think not demanding payment from the Council is part of Buffy's acceptance of Slaying not as a "job," which is how she frequently defines it in her high school years, but as a calling. Slaying is who she is, not what she does. And, as Xander points out, she's like Spiderman - action is his reward.

Of course, demanding Giles' salary was a selfless act. She rarely advocates for her own self-interest, and when she does, it generally ends badly. This is why Angel can charge people for saving people but Buffy can never quite get away with it.

And, you'll notice when she "joined" the initiative for five minutes, it didn't occur to her to ask for benefits.