I understand the dilemma. I wonder if they might be well-advised to post a sign at the ticket sales counters that there is a NON-Six Flags sponsored "Gay Day" September 18th and that way ticket buyers could THEN decide whether to go in or not. That way, "buyer beware" notice had been given.
This would simply end up in either instigating a conflict (anti-gay activists would descend upon Six Flags to create problems) or would result in lost revenue. Neither would be to the advantage of Six Flags (or any other park, company or establishment). The old adage "Let the buyer beware" seems to fit nicely here.