Maybe some are but I can say from personal experience that most of your traffic, if you're smart, comes out of a CDN. The sites themselves are definitely not that interactive which makes them simpler to publish. The pages are almost all cached and that doesn't take much horsepower to serve up. The big video sites have ratings and comments but they are not that big of a deal. People go to watch porn sites to watch porn, not interact. Customer analytics have shown that over and over.
I know of virtually no porn company that handles their own transactions, either. They all go through billing companies that handle things like PCI compliance for them.
Most sites also use a system like NATs to do their affiliate management. You need one that the affiliates trust isn't shaving sign-ups from their account. They tend to trust NATs.
For the data on the backend you just have a SAN to manage the data or you just manage it on a few servers with lots of disks but if you are really at the 100TB mark then you get a SAN I would think. That's what we did. Sure, it's a lot of space but they're big files so managing them isn't that hard.
I'd say the largest issue that a company like YouPorn will have is the amount of data in their working set for a CDN. CDNs generally charge you for the size of your working set that they keep at each POP in their network so you want to keep it as small as possible.
At the end of the day running a large porn network is more about integrating the myriad of partners you need to run the network. The infrastructure is interesting for a while but once you have it working the business of doing deals and handling promotions and figuring out why integration point A isn't working like it should is what keeps you busy.