“On bisexuality: It immediately doubles your chances for a date on Saturday night.”
                                        
                                        The earliest source located is  here http://books.google.com/books?id=kd41AQAAIAAJ&q=bisexuality#search_anchor, in the sidebar  "Quotations According to Woody Allen" http://books.google.com/books?id=kd41AQAAIAAJ&q=%22quotations+according%22#search_anchor which appeared alongside the New York Times article "Everything You Wanted to Know About Woody Allen at 40" by Mel Gussow, 1 December 1975, page 33. Full text also available in  Lakeland Ledger, 25 December 1975 http://news.google.com/newspapers?id=pUdNAAAAIBAJ&sjid=4foDAAAAIBAJ&pg=6578%2C6650273 on google news. 
Unsourced variant: "Bisexuality immediately doubles your chances for a date on Saturday night."