The anonymous element of it is obviously desirable to many in this instance.
Remember that anonimity in Bitcoin is
complicated.
Each transactions is recorded in the Blockchain. If I know both the address of the porn site and yours, and I see there is a transaction involving both of you, I can draw conclusions.
Privacy is not as straightforward as it might be advertised by some.
For porn, anonymity has not to be top notch. It is legal in most countries and people just need to avoid it being too visible.