See Ashcroft v. ACLU (II)(2004)

Essentially the government can’t limit the access to free speech (in this example: pornography) in the name of protecting minors beyond the least restrictive methods possible (e.g. parents of minors can already easily block access to these websites through many other software tools).

"adults without children may gain access to speech they have a right to see without having to identify themselves or provide their credit card information. Even adults with children may obtain access to the same speech on the same terms simply by turning off the filter on their home computers."