Whenever I read a speculative fiction or sci-fi story that includes a 'harem', I'm inevitably confused. Why are harem guards almost invariably male? (Okay, we can argue about whether or not eunuchs are male, but you see what I'm getting at.) Part of the point of the books I like to read is to present a new perspective on what it means to be human, and this is weakened if the author isn't conscious of his or her own cultural assumptions.
In this case, I think what's bothering me is the prevailing assumption that women cannot protect women. (Caveat: I'm only talking about my own culture here: American, middle class, post high-school, between the ages of 18 and 45, 1996-2008 edition. This is the only culture I've ever lived in, so I'm really not qualified to make sweeping statements that include more than that narrow range.)
When I think of what isn't shown in popular media when it comes to protection or guardianship, a woman effectively protecting another woman is at the top of the list. The only thing I can think of are movies like 'Panic Room', where one 'woman' is a child. We see men protecting other men all the time, although less often than we see men protecting women. In the latter case, we can usually count on some romantic tension, which helps sell movies/magazines/whatever.
What's going on here? Is this a lingering perception of inequality? Or is it just less sexy, and therefore harder to sell? Of course, saying we don't see it because it's 'harder to sell' isn't really an answer. If it sold, we'd see it. The question is why don't people want to see a kickass woman (plenty of those around, clearly we like those) protecting another full-grown, mostly competent woman?
PS: if you want to have your socio-cultural assumptions fucked with, go read some Octavia Butler.