> I don't understand why
OK, let me try to explain. I'm not against pornography. I'm also not a prude -- if consenting adults want to have orgies, or wife swap, or insert baseball bats into body cavities, or get slapped and yelled at, go for it!
I'd bet that 90+% or more of people's first exposure to sex is online videos. While there are certainly videos that show realistic sex, the vast majority are highly unrealistic and set incredibly weird expectations about what is normal. A lot of what is shown on video is bad acting, not happiness.
To distinguish itself from from other videos, makers are always trying to find some new or more extreme boundary to push. A lot veers into extreme power dynamics -- boys learning about porn should not have the expectation that women like to be choked or spit on, or have jackhammer anal sex. Girls shouldn't have the expectation that that is what boys expect of them.
As an adult you might say, hey, a lot of those scenarios are just entertainment, eg, those step siblings are obviously in their 20s, or obviously women don't like having 100 men ejaculate on them, but to children where this is their first exposure to sex, they absolutely don't know what is normal and this imprinting causes harm for some.