They generally make money off subscriptions ann ad programs, most of the time when you click the video they'll do one of 2 thinkgs. 1. They will only allow you to watch a number of clips a day, after that you have to pay a subscription or V.I.P. fee to watch unlimited clips and have full access to their video library. In addition all those adds are placed for a reason. Most user will accidently click the ad and what ever ad program they are running will count that and boom. they are brinign in the income. 2. They solely rely on the ad program (most common for free adult sites) they will even put videos that when you click the video it clicks the ad at the same time. Really tricky business but can be profitable
Joining affiliate programs can be very beneficial for adult sites. Some may give content for you to use, so that it entices users to purchase memberships on the affiliate's site.
many adult sites earn money from membership fees,you want watch video ,you need to pay the annual fee to become a member.
There are many different ways that you can make money in the adult market. From being an affiliate, blogger, photographer, hosting company, producer, advertiser...etc. What specifically are you looking to get into?
Some charge for membership in order to access site content, others sell adult products/services, and some do both.