[hnbad]

In addition to what others have said:

* Porn is almost always posted in violation of copyright.

* Hosting porn opens you up to legal issues if you can't verify that everyone involved is adult and consenting.

* Payment processors, hosting companies and other service providers you rely on usually have strict policies excluding porn.

And that's just addressing legal pornography, not other "NSFW" content like child abuse, animal abuse, general violence or gore. If you run a large enough public user generated content service people will use it to distribute illegal content or flood it with jihadist execution videos to ruin someone's day.