Why is nginx so popular with the adult websites?

Because nginx is really good at streaming, on a huge number of connections, simultaneously.

Meaningless question without an idea of how many of the top 100,000 sites are adult sites.

Agreed. The data is likely to be skewed when the majority of the internet is porn.

You can compare this with say Apache use in the same context on the site

Because they like making money. To make more money you serve more users with less hardware. It's a pragmatic decision.

Maybe porn world is more agile than the rest, and thus are able to choose more modern solution.

The fact that it ends with the letter "X".