I don't have a problem with OP's approach and I think he should be free to put whatever he wants on the content-stealing sites, but he didn't have to make it sexual.

A very annoying, loud, visually busy animation would've sufficed. Baby shark at maximum volume, or a continuous fart sound, or maybe just a high pitched beep would also sufficiently scare away people.

By showing porn in a place that he knows minors and other protected groups will visit, they violate decency laws in a whole bunch of countries. They probably wouldn't if this was accidental (i.e. they sold their domain to a third party that turned it into a porn site) but in this case they admit this was very much intentional.