(Not to be taken for granted in a genre increasingly heavy on dialogue with the cadence of jokes, but not the punchlines.) For one night a year, however, the mandate is suspended, and the horny unwed are legally permitted to get it on. It’s National Sex Night, after all, which appears to have become something akin to summer Christmas, except the stores run out of condoms instead of turkeys. If “One Night Only” is aiming to comment on the hollow, ineffective nature of much contemporary political activism, then job done. Yaron Orbach’s lensing makes an airy, lacquered lovers’ playground even of a hot August night; the film gives us a postcard New York that at least looks somewhat lived-in.