While people can certainly still go to brick and mortar stores and purchase pornography, for the most part, the overwhelming majority of pornography, whether purchased or viewed is now done over the internet. This does present several problems from both the viewer/purchaser perspective as well as the actual website/owner operator. For instance, most people viewing or purchasing porn, no matter how the porn is labeled, presume that what they are viewing or purchasing is legal to do so, especially if they do not download, copy, or purchase anything, but only view it. This viewpoint is not legally correct, as the First Amendment does not cover or protect you from viewing child pornography or things that are judged to be "obscene". In regards to obscenity, for the most part, prosecution authorities typically do not pursue cases of this sort, for whatever reason, although the laws on the books allow them to do so. As for child pornography, at the present day, this is aggressively prosecuted and appears to be the only area of the porn industry that the justice system consistently acts against.
In regards to the actual website that provides the porn for viewing and/or purchase, with the exception of the basic boilerplate legal notice disclaimer, if you are of age to view porn, you may do so, without having to show demonstrative proof that you are of age. This means that the barrier of entry for the viewer is virtually non-existent. Additionally, while there are adult web sites that have legal disclaimers on their porn in regards to having verifiable proof of the legal ages of the performers in the videos that are being viewed, the vast majority of porn sites, have no such disclaimer or legal notice, whatsoever embedded into the video. This means that the viewer or purchaser of most porn online is assuming that everything that is being viewed or purchased is both legal as well as being authorized, but that assumption is often misplaced.
In point of fact, porn web sites, should have to prove a couple of things to their viewers, such as first that the sexual actions being view or purchased are legal to begin with, which means, by definition, that they cannot be hosting or storing any child pornography whatsoever, and additionally that all images/videos being shown/sold are not obscene and legally authorized. Also, and very importantly, each video clip should state that the performers in it are of age, that there are records that prove this, and further that these performers have each provided written and expressed permission for the usage of, the reproduction of, the distribution of, or the exhibition of this particular performance for the time period, so indicated.
There isn't any real good reason for the government to assume that these hosting websites showing and distributing porn have the legal right to do so, where often, that legality would appear to be seriously in question. The thing about digital media is that it can be copied, edited, re-posted, and modified, ad nauseam, so that the actual ownership of such product or legal right to distribute it, is murky, at best.
The porn genie was let out of the bottle, a couple of decades ago, and in the internet age it has morphed into something that lacks any semblance of control. However, there can be control in the porn world, and that control begins with the enforcing of basic legal copyright laws such as: who owns this, who has the rights to this, and what are/are not the distribution rights to it; and those that cannot prove satisfactorily that they have said ownership rights or authorized permission, and further are not in violation of Constitutional law, should be put out of business or held accountable.