I have not personally been to a Gay Day at a park (have you?). But I have checked various websites orientated around Disney holiday planning. Among them are posts of heterosexual visitors who had their families at DisneyWorld during Gay Days. The statements I read (mousesavers.com is a really good site for money saving strategies and reviews) from people who were there, say that unless you KNEW that it was Gay Days, you wouldn't notice it.
But, again; you may check the Six Flags website (www.sixflags.com/parks/fiestatexas/index.asp) to your heart's content. You will not find mention of Gay Days at all. Same thing with DisneyLand, DisneyWorld, Magic Mountain or Busch Gardens. If a homosexual group decides to call, and reserve blocks of rooms at the park; the park has no choice but to accept the reservations. Whether it's a church group, a high school event, a class reunion, or even NAMBLA. The park has no legal options but to accept the reservations. Whether they agree with the group, despise the group or are totally neutral; the laws of the land are quite clear.