Allow me to chime in here, please.
When I was in graduate school I did my thesis on the Law and the Internet. This was 15 years ago when the 'Net was just coming into existence. One of the chapters discussed the subject of online pornography.
The most puzzling question I encountered in my research was, for the purposes of my study, to come up with a definition of “pornography” and, putting morality aside for a moment, I discovered that there is no legal definition of pornography; it is largely in the eye of the beholder. In
Miller v. California (1973) even the U.S. Supreme Court could not come up the a working definition of pornography so it left the decision up to the individual localities, essentially saying “If the material violates your local community standards for decency and those communities want to regulate it, you have our (the Court’s) blessing.”
Justice Potter Stewart, in
Jacobellis v. Ohio (1964) remarked that he too could not define pornography but (paraphrasing) “[he] knew it when he saw it” and regarding the subject matter at hand, a French film called “The Lovers”, (a story about an adulterous love affair), was not, in his opinion, obscene even though it contained nudity and a scene of adulterous sexual intercourse.
On the other hand, I remember when I was in Catholic school back in the 70s, the Sisters were quick to snatch the annual Sport’s Illustrated swimsuit issue off the racks of the library because it was “obscene.”

Such stuff is fairly mild by today’s standards.
In my humble opinion, “looking” at “pornography” - however you define it - may or may not be “sinful” depending on the context of the situation. I do not consider my thesis research sinful because I considered it to be for academic purposes; not for sexual titillation. I am also a computer technician by trade and occasionally I have to remove malware and viruses that connect to sexually explicit websites from people’s computers. I may be “looking” at pornography but don’t consider that to be “viewing” pornography (if that makes sense). However, sneaking a look at a stolen Playboy back when I was at Boy Scout camp probably was wrong.
So in conclusion, looking at “porn” or “erotic” art for the wrong reasons (whatever “wrong” means) is probably sinful. Looking at it for academic, scientific, bonafide job-related reasons, or artistic appreciation is probably not.
My 2 cents.