Adult seo backlinks/blog posts

I guess the best is to make an adult google "friend" site and sell ad spots

Adult SERPs are typically a lot less competitive than you would think. The major barrier to entry for most topics is competing with legitimate sites that have lots of social metrics and editorial links. In adult, all link building is pretty much done with link trading and buying because nobody is going to share porn on Facebook or link to it in news articles, etc.

The other thing is most adult sites spread their juice reeeeeally thin over thousands of pages of content, so if you see a redtube video ranking #1 for a keyword you can assume it's only there because of DA. It actually shouldn't be hard to outrank them if you build a more focused site.