JUST when you thought American culture couldn’t sink any lower, along comes Fox TV’s “Who Wants to Marry a Millionaire?” During the two-hour show this past February, 50 young women vied in a beauty contest, which included a swimsuit parade, to marry Rick Rockwell, a California real estate developer, motivational speaker, and aspiring comic. Rockwell picked from the pageant of lovelies an emergency-room nurse named Darva Conger, went down on his knee, and when she accepted, married her. (Since this was Las Vegas, the producers were able to have a license made out in the five-minute commercial break between the proposal and ceremony).