No need for that. Seriously, 99.99% of the time you have enough information in the title, series description and the scanlators site to tell. And... euh, just don't read hentai if you're offended by :
- Rape
- Incest
- Tentacles
- underage sex
- zoophilia
- etc...
And if you're going to start with these stuff, we're in for a lot more trouble. I mean, we'd eventually have to filter Guro because people are offended by it...
And then some people find manga involving angels/demons and stuff religiously offensive and then who knows what else some people can find offensive...
Actually, in short? I agree with NeoFireHawk. (euh and I think Kraust too).