They can skip recurring memberships, and charge by video or image set.
This also makes it more difficult to do a siterip, where someone buys a single month membership, downloads the entire site, and cancels it right away. If someone wants to siterip the whole site, it would cost them hundreds, if not thousands of dollars.
Alternatively, switch to 3/6/12 month memberships only.
Had porn sites accepted bitcoin early on, and kept the bitcoins, well, shit, they'd be sitting on a huge pile of money that would absolutely dwarf any recurring membership.