Yes, it absolutely is. Pornography and masturbation to unclean thoughts is a sin. It is absolutely. Do not let anyone fool you with tricks in their words.
What part are we to have with uncleanness, if we are to be apart of the Kingdom.
Is it difficult to stop? Yes, and it becomes more difficult the longer you do a thing. But you can stop, you can pray to God and ask for the power to stop, not just stop looking at Pornography, but to stop having lusting thoughts and fleshly desires that push you to look at porn.
People will lie to you, as they lie to themselves, because they love their sin, and the pleasures which come from it.