St. Patty's Day is anti-gay?

I just read in an NY Times brief that NYC's traditional St. Patrick's Day parade, for which they turn the river green and march around supporting Irish heritage, doesn't allow GLBT's to march under their pro-gay banner. The Irish-Catholic organization responsible for the decision is the Ancient Order of Hibernians. But why does the parade's sponsor feel the need to persecute on the happiest of days? Why does it matter, especially in our nation's most populated and most diverse metropolis?