All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Allah, The Exalted, Has Commanded believing men to lower their gaze, where He Said (What means): "Say to the believing men that they should lower their gaze and guard their modesty: that will make for greater purity for them: And God is well acquainted with all that they do. And say to the believing women that they should lower their gaze and guard their modesty" {An-Nur, 30-31}. Allah, The Almighty, Has also Prohibited all kinds of indecencies and temptations and whatever leads to them. It goes without saying that pornography sites are among the gravest causes of evil which leads to committing these taboos, and they are, unquestionably, prohibited.
Not only this, but the degree of prohibition is severer for those who facilitate for the internet surfers to access such sites because, on the Day of Judgment, they will bear their own burden and that of those whom they have misled. Allah, The Exalted, Said (What means): "Let them bear, on the Day of Judgment, their own burdens in full, and also (something) of the burdens of those without knowledge, whom they misled. Alas, how grievous the burdens they will bear!" {An-Nahl/25}. Moreover, Prophet Mohammad (PBUH) said: "If anyone calls others to follow right guidance, his reward will be equivalent to those who follow him (in righteousness) without their reward being diminished in any respect, and if anyone invites others to follow error, the sin, will be equivalent to that of the people who follow him (in sinfulness) without their sins being diminished in any respect". {Muslim}. And Allah Knows Best.