They're not there to persuade anyone; like Critical Mass or Dore Alley, these aggressive displays of difference just reinforce the gulf of "we're totally unlike you," rather than adopting the shrewd Scientology approach of "hey, look, this is something that you've been seeking all along."
But the problem isn't their message -- it could be about global warming or shoe stores or a dramatic reading of for all we care -- it's that they're being loud, and we're just trying to enjoy a nice day in a public park for Pete's sake. But they don't mean to be making anyone grumpy; they really think they're doing everyone a favor. They think they're being nice. Meanwhile, everyone else just wants them to go away.
So: what's to be done? You could ask them to stop, but just as with noisy neighbors, they don't care that they're irritating; if they were polite, they wouldn't be making a racket in the first place. Or you could blast them back with a loudspeakering of the "Every Sperm is Sacred" song, a sort of mutually-assured deafening. Of course, that's not likely to make them leave.
We're partial to the "if you can't beat 'em, join 'em" approach: make them uncomfortable by holding signs reading "God is Goddamn Great," and giving highly detailed testimony about how you found Jesus during the physical act of love. Hey, it's a public space: you might as well play along until they just want to take their toys and go home.Sixteen Candles