That is like asking if Hollywood movies are pornography. Some is, but most isn't. Blockbuster and some Suncoast Motion Picture Company locations put stickers on nearly all anime, they put the sickers on to limit their liability, as they don't want people suing them for trying to warp their child's young minds, but in my opinion, any kid with parents like that is already warped, they don't need Suncoast's help. Honestly, I don't know. I have never rented or bought hentai material, and I believe it is easy to avoid accidentally buying or renting.
Rental stores worth their parking spaces keep the adult stuff in a separate section.
There is a fair amount of nudity in anime, yes. There isn't sex in anime unless it was intended for older viewers. I have managed to stay away from the titles that have sex in them, but due to cultural differences, there is more nudity in anime than one would expect. Americans have grown up with the idea that animation is children's stuff, as well as the idea that nudity will destroy and warp your mind. In Japan, genitals are pretty much illegal to show, in animation or live action, so anything that would be drawn is blocked over.