The biggest reason is that the general assumption is that one female in a room full of males will use that to her advantage. Just as one male in a room full of females would likely do the same (though also likely to less effect).
When you know you can get attention and special treatment, you're more likely to strive for it. That's the generalization of females on the internet.
Do I agree with it? No. Though I would be lying if I said I had never witnessed female players who lived up to that stereotype. In fact, I have known a few in my time playing this game.
My guild has a massive amount of female raiders. Even compared to most of the guilds in this thread who have three or four. My guild has, as far as I can count now based on future interest and previous roster, no less than six. Only one of them, funny enough, is a healer (and one of my most trusted healers to boot, as I am the healing class leader). One is a tank/melee DPS, three are melee DPS, and one is a caster DPS. All of them are fun to be around. Sure, I am flirty, and they are flirty sometimes, but it is respectably so and not in the interests of "hooking up" or "manipulating for gear" or anything of that nature.
I can understand the stance that some guilds might have against female members. If I recall, there's even one guild out there that has a strict "no boys allowed" policy. Some people just don't want to take that chance. I don't agree with it, but I can respect that decision.