The DVD marketplace is split between those providers who rent adult DVDs, and those who don't.
The only time this (potentially) becomes a matter of concern is if the whole family might be accessing the membership, in which case you might feel more secure using a DVD rental provider which doesn't allow you to rent adult DVDs.
DVD providers with family-friendly, no-adult-DVD policies include Blockbuster and Tesco.
DVD providers which do rent adult DVDs include LOVEFiLM and MyMovieStream.