HBO is showing a documentary for Valentine's Day that truly marks the occasion: "The Loving Story" tells the drama, and the love, behind one of the more remarkable court battles of the 1960s. Richard and Mildred Loving were the interracial couple who enlisted the assistance of the federal government and the ACLU when Virginia officials told them their marriage was illegal under state law. The Supreme Court eventually struck down the statute. The documentary airs tonight. A docudrama made in 1996, Mr. and Mrs. Loving, starred Lela Rochon and Timothy Hutton. It is available used from some dealers.