In short: it’s a massive pain that comes with responsibilities other content doesn’t.

Hosting porn is a responsibility nightmare. You need to have a system in place to verify that all people depicted in porn are consenting adults who agree to having content hosted with you. Pornhub killed most of its catalogue to implement such a system and competitors are getting sued for not doing the same. Movie theaters have shown porn movies that have later turned out to be rape, and the porn industry is full of videos depicting actors or actresses that did not consent to beforehand. Think “casting couch” except someone who thought they were getting into modelling gets getting dragged into a room right after signing a vague contract. The studio will say it’s all an act, but that’s not what the victim says when the video comes out. Luckily, hosting copyrighted content is already illegal, so you can squish that problem before it becomes too big a deal.

Next, you need to differentiate between porn and revenge porn. There are organisations and companies that focus on helping revenge porn victims get their shit offline, and if you’re hosting porn, you could save yourself a lawsuit or bad publicity by working together with them. That’s going to cost time and money, but it’s better than the alternative.

Then you need to make sure that the porn that gets uploaded isn’t child porn. There are APIs to check uploads against known, existing footage, but you still need to be ready to take action if content that law enforcement didn’t know about gets uploaded. One or those tasks is judging whether or not the people depicted in that type of porn are adults or not.

Of course you’ll need to follow local laws. How many small Lemmy operators followed the law when some douche spammed child porn on the shitpost community? I doubt many of them read the law before purging the images! Some places demand you keep an isolated copy for law enforcement, but any sane person would want to purge such content from their servers as quickly as possible.

Any hosting service needs to be protected against CSAM, but when you set up a site people come to jerk off to, the probability of this trash getting uploaded becomes a lot higher.

Then you need to implement measures to prevent exposing minors to porn, because that’s a crime in many countries. That means registering age/date of birth and hoping nobody sues you because it’s easy to lie about it. Most websites use 18 as a cutoff for access to porn, but not every country considers 18 to be the age you can watch smut.

Apps need to be sabotaged because major app stores don’t want you to see porn. Apple is more strict in this than Google, but you’ll still need to watch out. If you accidentally break your filters or a mistagged post appears during review, you’d better have a good explanation to the reviewers about why they should let you publish your app.

Then there’s the business side of things. Puritans may get pissy, but that’s the least of your troubles. All major international credit card processing companies have either explicit or implicit terms that state you can’t do business with them if you host porn. This is because a) these companies are your typical puritan American big business and b) people who buy porn often get found out or regret their decision, which is followed by a fraudulent charge back, and that costs you and the processing company money. If you’re taking money from paying customers, you’d better read the fine print because you may just be cut off at any random moment in time.

If you make money through ads, you either use one of the few companies that accept porn (and make every ad on your site a gambling ad or a porn ad) or you figure out how to comply. Perhaps you can disable advertising around porn, or lie about your policies and hope nobody over at Google double checks.

Onlyfans makes use of weird rules about art and other technicalities that expensive people came up with. Platforms like Patreon nearly got screwed over for allowing porn. It’s possible to make money with platforms allowing porn, but it’s certainly not cheap or easy.

Lastly, there’s porn laws. Japan, land of the tentacles and schoolgirls, has strict laws forcing genitalia to be censored. In the UK, spanking and face sitting is considered illegal. In some countries porn is banned in its entirety, in others there are laws government drawn porn (hentai/rule34) and more specifically if drawings of children constitute child porn. In some American states you need to do special identification to make sure your visitors are adults. If you sell access, there are special tax rates for porn that differ per state/country.

The solution to all of these problems for most sites is to either ban porn so they don’t need to take special care, or to have special cases for porn with dedicated rules. Usually the most liberal of American states is used as a reference point and from thst point onwards you cross your fingers and hope nobody sues you, and that your holidays abroad aren’t cut short by a border agent checking your history.