Free porn sites serve to MAKE MONEY. 

You can protect your material and create your paid areas, but that does a lot of work. Most webmasters choose the simplest method , ie advertise one or more sponsors. And the best method to sell sponsors is free porn sites. 

The sponsor must be fit for free site content if the site has photos of girls in bikini it is better to find a sponsor who gives access to thousands of photos of girls in bikini. If the free site is dedicated to high heels, then enter as sponsors sites that handle high legs, stockings and high heels. 

An alternative to adult sponsors can be non-adult sponsors, such as online meetings, software sales, online casinos, etc ...

It is best to limit only 2-3 sponsors to each site. Then it is at the discretion of the webmaster, some put different sponsors, others only one that converts them better. 

Always read the terms of service of the sponsors you will advertise. If you use non-adult sponsors, verify that you accept signup generated by adult sites. Review your traffic statistics and sponsors to try to maximize your sales. 

You have full freedom in how to create your own website. If you choose to enter sponsors, be sure they are fit for your particular website and content.