These groups argue that states offering these plates are violating the First Amendment by engaging in viewpoint discrimination -- allowing an anti-abortion message but not a countermessage from the abortion rights side. They also argue that the plates are a church-state violation, because the words "choose life" are found in English translations of the Bible.
The church-state argument is about as unserious as legal arguments get. "Choose Life" must be banned because the two words are found in an English translation of Deuteronomy? If so, the license plate message "Don't Kill Anyone" would have to be banned as one translation of "thou shalt not kill." Plates saying "No Capital Punishment" could be removed as sectarian, too, since the Catholic Church opposes the death penalty and is loudly pushing that message.
Besides, the assumption that opposition to abortion must be religiously based is wildly erroneous. Anyone who thinks this should check the anti-abortion writings of two of our best known atheist journalists, Christopher Hitchens and Nat Hentoff.
The viewpoint discrimination argument is more serious. The government would be guilty of viewpoint discrimination if there is demand for pro-abortion plates and states refused to meet that demand. But as plaintiff lawyers will tell you, the abortion groups do not want their own plates. (What would they say, "Choose Death" or "Stop That Fetus"?) They want the other side's plates to disappear. This is an argument for censorship decked out in the language of free speech.
A sounder argument against these plates would be a comprehensive one: that the state has no business featuring slogans and viewpoints of any kind on its plates. No commercials for sports teams, professions or the Sons of Confederate Veterans. No advice to ski in Utah or to live free or die. No plates celebrating the AFL-CIO or NRA (both available in Virginia). No political guidance on environmental concerns or abortion.