who puts advertisement on a porn video?

The whole idea of most every web site is to either sell crap or to get attraction to a web site for the purpose of seeing advertisements. Just look at Similar Worlds website. The owners aren't providing this place simply for our enjoyment... they are making money from all the advertising you see all over it, unless you are a VIP that has PAID them to not see the ads.
Porn sites are the same. They sell tons of advertising or get users to become paid members to access content. And then they make even more money off of selling your contact information to other advertisers.