Simply put, it is no longer the 1950's. The tide has turned in the culture wars, and homosexuality is not going to be cause for persecution anymore. Excluding gay groups from the St. Patrick's Day parade will not accomplish anything other generating negative press for parade organizers. Furthermore, it cost them one of their largest sponsors, as Guinness withdrew their support rather than be aligned with exclusionary practices.
Gay groups, and other LGBT groups, have just as much of a right as any other groups to participate in parades. Excluding these groups marginalizes them and categorizes them as an "other" that doesn't deserve a spot with other event participants. Letting them participate raises awareness and normalizes their lifestyles..
Yes, gay groups should have been allowed to carry signs in New York St. Patrick's Day parade, because it is just a matter of fun expression. If that makes a gay group happy on St. Patrick's Day parade, by all means they should be able to carry their signs. It is only meant to be a fun holiday.
I do not believe gay groups should have been allowed to carry signs in New York's St. Patrick's Day parade. I understand the LGBT rights movement is very important and I support it, but I think it needs to find its own forum. The parade should bring the community together. It shouldn't be a forum for disputing such things.