The novelist Lorrie Moore, writing about that new movie with the very long lesbian sex scene, "Blue Is the Warmest Color." Let me also excerpt what Moore says about the main actress's mouth:
In general Adèle’s soft wide mouth hangs open throughout the film, revealing an attractive overbite long associated with French actresses. She pulls her hair up, lets it fall again, ties it back up—continually. Between the slack mouth and the unstable hair, we see quickly that Adèle does not quite know who she is. But she is a creature of appetites, and much time is spent watching her pliable mouth chew—pasta, candy, oysters.I so much prefer watching those words to watching whatever that looked like in the darned movie.
Presumably, the mouth, being part of "heart’s great canvas—the face," has more to say to us moviegoers than those nether lips that are so dull in the tedious sex scenes, and yet Moore makes all that mouth action sound boring too (even as Moore's prose is not boring). Which is why we read. And that's a message that one must assume that a novelist writing about movies would like to convey.
ADDED: Moore says that "most long sex scenes" are "emotionally uninformative, almost comedically ungainly and dull to watch" and adds the parenthetical: "Did we learn nothing from Vivien Leigh’s little morning-after smile in Gone With the Wind?" How could genitalia compete with that mouth? Vaginal lips have nothing to say.