New York Mayor Ed Koch, an early opponent of the $7 movie ticket in that city, said over the weekend he will still attend movie theaters charging that amount until there are more people behind the drive to change it. Koch, who called for the boycott last week, admitted he went to a $7 movie Thursday night--"Ironweed," starring Jack Nicholson and Meryl Streep. "We'll pick them off, one at a time," Koch said. "I'm saying, let's flex our muscles. Write me a letter at City Hall. We can bring the movie moguls to their knees, to their knees." "That's a disappointing statement for the mayor to be making," countered Robert Sunshine, director of the Metropolitan Theater Owners Assn., which represents about 125 cinemas in New York City. Sunshine added that the hike to $7 was needed in many cases to offset higher rents and renovations. The mayor also made his public debut as a film critic when he added, "Ironweed isn't worth $5."