LONDON (Reuters) - British pop star Elton John plans to marry his long term partner David Furnish on December 21, the first day that civil partnerships between gay couples will be possible in England. ADVERTISEMENT In an interview in Attitude magazine, which hits the newstands on Friday, John said the ceremony would be a small, private affair involving only himself and Furnish and their parents acting as witnesses. "Out of respect for their (our parents') support, we want to just keep it small -- not to make a ballyhoo of the ceremony," he told the gay lifestyle magazine. "There will...