This is likely The Mating Habits of the Earthbound Human (1999).
David Hyde Pierce (of Niles Crane /Frasier fame) was the "Narrator"
An anthropologist from an alien planet provides voice-over commentary for a documentary look at human courtship, mating, and reproduction: "complex, perverse, tragically beautiful: the earthbound human." The commentary gets some of the details comically wrong (explaining the slap on a newborn's back: "they don't like the looks of the child so they beat it"), but gets right the overall idea. We follow Billy and Jenny from their meeting at a club, initial misunderstandings, his losing her phone number, finally connecting, walks on the beach, soulful talks, meeting each other's parents, friends' advice, and the gradual taking of the relationship to new levels.
Hyde Pierce, acting out the role of an alien (credited as "infinity-cubed" in the opening credits), narrates a courtship in a late 20th-Century American city, presenting findings and opinions as an extraterrestrial nature documentary. The relationship "footage" is played straight, while the voice-over (with its most often wildly inaccurate theories) and elaborate visual metaphors add comedy. Among other themes the film explores the possibility that, whenever describing their interpretations of other species' acts and feelings, humans may get everything wrong.