The salient aspect of conventional "lez" porn is that the women are captive in the frame. If the two of them drove off together into the sunset, out of camera range, it would be a violation of an implicit code.
As with most porn, the viewer derives satisfaction via an exclusive window into the private, explicit activities of the participants, but the key is that the behavior of said participants is at all times subject to the whims of the viewer, in that one retains control over their activities by virtue of one's possession of the remote control. At the moment one switches it off, they remain frozen-in-time in the frame, ever-involved in the activities they portray for the benefit of the watcher, ever-at-the-ready for the next viewing...
This situation makes possible a "lez" film context in which a het-male viewer can obtain the triple benefits of a) two writhing bodies instead of one, which provide added opportunities for excitement, b) an ostensible didactic or instructional element, "stolen" and secreted away by the viewer owing to his privileged sneak-peek into the behavior of women "amongst themselves", and c) the authoritarian and perhaps even paternal satisfaction of "permitting" said activities sans negative consequences (cf first paragraph above re. lack of participant agency).
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/ dave /