Generally on those occasions when I've known my kids were reading something I thought might have "mature content," I've talked with them briefly about how some of what they read might make them feel uncomfortable, and if so, to talk with me or someone else about it. I don't worry much about sex itself (both my kids know quite well where babies come from), but violence, particularly emotional violence, e.g. manipulation. As I've commented to my kids, there are a lot of different ideas out there about sex, and I'd feel better if they get used to what are (ok, in my opinion, anyway) healthy kinds of sexual behavior first. I suppose this is just another kind of prudishness, but hey, it's part of being a parent-- you have to try to figure out what's best for your kids to help them be able to grow up and take care of themselves as adults.
My younger daughter generally asks my advice before buying manga, for example. I had to define "yaoi" for her once-- she decided on her own not to buy that one. (Mostly because she doesn't much like romance of any sort.)
My older daughter is in a rebellious stage, which is too bad, because I do see her getting upset about some stuff she watches or reads, because it makes her uncomfortable. But after a certain point, kids get beyond being willing to ask for or take advice, and she's 17.
