The “oh it’s just the same as cave paintings”brigade are disingenuous, to put it mildly.
People have always depicted others having sex, with whatever technology is available. It's just that it's only been in recent years you don't need a lot of money to make moving images, because the technology is now widely available very cheaply.
If your watching images, you don't have to do much work, just watch. If you're reading, the mental load is more, as you have to do the imagining. If I write, "a man in a black hat walked down the street," everyone reading will imagine different streets, different styles of hat, daytime, nighttime, modern day, different historical eras, men of different ages, heights, builds. On screen, you just get the man in a black trilby walking down a dark unlit street with no other people about, or however they chose to depict it. There are things you don't know, like whether the actor chose to be there, whether they had any input on the direction, whether they are getting paid, and if so, how much. We might know from other sources, and that's how we know many of those in porn films have had little choice in the process.
That can happen in written fiction, too, but the difference is, no real people are harmed. It could inspire people to harm others in real life, but I suspect not quite as easily as seeing it on film would.
I've never read de Sade, but I doubt he was the first person to enjoy sadistic practices; he was just the first to result in an eponym, because of what he wrote. I don't know how influential he was on sexual practices (wouldn't be surprised if someone's done a PhD on it somewhere, though.) He was influential enough to inspire the teams sadistic, sadism. We know writing can be influential. I don't think it's influential in the same way as film - it works on different bits of the brain. They can both be problematic.
And historical context is always important. It usually tells us things aren't new, and if we ban them, they don't go away - they might become far less widespread, though.