Remembering a delightful photo I'd seen circulating the internet over the years, I immediately asked a nearby woman who was happily counter-protesting if she could quickly whip up a sign that simply said "GREAT" on it, and then would she be so kind as to hold it up directly under/over the "HOMO SEX IS SIN" sign?
Much to her credit, the woman smiled and hurried off to make it happen. This is the resulting picture.
It's always deeply tempting to negatively engage with people espousing such hurtful bigotry - but, in my experience, nothing is ever really accomplished by that save making the other person feel more righteously justified in their cause. Crazy queer is attacking me! I'm right, they're all dangerous anger bears! Which isn't really what we should be aiming for, right?
Better to go for the good-hearted laugh, I say, and then focus on more long-term, educational, compassionate campaigns to change public opinion overall. Expose these folks to real live gay people - people from all walks of life, regular folk just trying to be themselves - and kill 'em with kindness (and humor), as they say.