They're based on your internet browsing. Not a whole lot you can do about it. If they're really inappropriate, I think we can ask that they not display that ad, but in general, it's based on what you view online.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry