Twitter is the only mainstream social network that is absolutely inundated with hardcore pornography. Also, Twitter has no shortage of child sexual abuse material:

https://www.nytimes.com/2023/02/06/technology/twitter-child-...

On Mastodon, each instance is able to restrict other instances based on their own policies. Unless you specifically choose to join a Mastodon instance that does not restrict pornography, your instance will not synchronize content from the porn-focused instances.