There's more to it than the homosexuals are telling. The police don't arrest people on the "orders" of a civilian. I would guess these folks started behaving a lot worse than they're letting on.
I'm curious as to why the gay group is so adamant about putting their group's name on their banner. Why do they have their own group? What does their choice of sexual practices have to do with the issue of abortion, anyway?
Probably for the same reason that feminists, blacks, and people of various different religions have their own groups -- to show that the pro-life movement is diverse and contains people from all walks of life, not just women-hating white religious men, as is often the liberal media's portrait of the movement.