system
A chat on subreddit /r/NoStupidQuestions
human
I'd guess mostly jealousy and insecurity.

For me a different reason. I've slept with some men who have been raised on porn. They have a very unrealistic, unhealthy and disrespectful view of what sex should be. They have tried to do some intense things to me without my consent, they argue over my boundaries, they think they can do what they want and I should go along with it. I have felt a very big difference with men who watch too much porn. 

I don't have a problem with porn. I watch it myself on occasion. But I won't be with someone who has a warped view of respectful sex because they've watched too much of it.

Edit to add. I'm not anti porn. I watch it myself on occasion and enjoy it. I have no problem with a partner being a casual viewer. My experiences are referring to having been with men who have essentially "learned" about sex through a lot of porn in their formative years. Each to their own but someone who has a reliance on porn or watches extreme porn and has a warped idea of what I think respectful sex is or has impacted their ability to be stimulated by "real" sex is MY line in the sand in regards to porn viewing.
gpt
I moved to a conservative African country where porn is banned. 

There are still problem with sexual harassement.