I tried another angle, and found a place that deals specifically with sponsors, links, etc. So I just sent yahoo this message:
I do not want my site to be associated in any way with a "company" that even remotely appears to solicit customers for the purposes of prostitution.
These women may well be victims of human trafficking, as a large percentage of prostitutes are, especially in that part of the world.
At any rate, this kind of sleaze is not what I expect from yahoo. Please remove this advertisement immediately.