I see it as paying for a particular amount of time, in which to enjoy various sexually related activities. So I don't see it as just paying for the actual act (whatever type of sexual act that may be), but some time leading up to it, and relaxing afterwards. And then perhaps repeating, or having a go at something else. I therefore expect to receive that length of time, and have done so far.
It's actually the only thing that I can think of where I do pay for a particular length of time for anything. Yes, I may pay for other things by the hour (or part of), but that's paid for the length of time the job takes to complete. As an example, I wouldn't be booking an electrician for 2 hours to install a new socket for me; I'd book them to install the socket, then pay for the time it takes to complete the job.