No End in Sight
The vast majority of pornographic movies, books and stories end with no ending; just an iris out or a fade/cut to black, while the action—implied or explicit at that moment—continues. There is no cessation of sexual activity, no implication that the characters have simply gone through an unusual phase of their lives—the level of sex they are engaged in is permanent, and all the viewer can do is simply stop watching after a certain point.
Although to one degree or another this has always been the case in porn for as long as it has existed, it has become overwhelmingly predominant since the 1990s—possibly driven by the market's easy access to short, free adult content on the Internet, the producers of full-length pornographic films have shifted from at least token efforts at story-telling and plot to Porn Without Plot "anthologies". In some extreme cases, such as the work of Andrew Blake, individual scenes become little more than set pieces, with a focus entirely on the act to the exclusion of such niceties as characterization, context, or even continuing the action as far as orgasm (itself a definite subversion of the earlier trope of the Money Shot).
It has also been argued that the implication of a cessation to the characters' sexual activity, however temporary, would act as a Downer Ending that would turn off the reader/viewer.
Compare Offscreen Inertia.
No examples, please; We already have Porn Without Plot for that.