“The Crime of Being Married.” So read the headline that accompanied photos of Richard Loving, a Southern “white trash” construction worker, and his African-American wife Mildred in the pages of Life magazine. One day — maybe today — audiences will sit down to watch Jeff Nichols’ nobly hatred-proof period romance, “Loving,” which goes nearly the entire first reel before explaining that mixed-race marriages were illegal in the then-segregated Virginia of 1958, and they’ll be surprised to learn what the crime in question was, having already observed and accepted the on-screen couple without the blinders of racial prejudice.