Social networks require a significant investment of dedicated resources. They generally require active development and maintenance, require server and hardware investment, and customer support requirements.
That being said, there are at least two open source platforms that I am aware of, but I do not know if there are carve-outs for adult content:
Mastodon is the most popular:
https://github.com/mastodon/mastodon
OSSN:
https://github.com/opensource-social...socialnetwork/
Lemmy:
https://github.com/LemmyNet/lemmy
You could also build one from scratch using any number of existing language-based frameworks. I've done this for clients in the past.