The billboard "aims to educate the public about two of the lesser-known aspects of the Mormon faith: First, that black people were forbidden from entering the priesthood until 1978; second, that sexually-active gay people are still banned from the church altogether."
(Virtually all white adult male members held the priesthood in the LDS Church. It is required for participating in certain spiritual events as well as for holding significant leadership roles. Priesthood is also required, according to church doctrine, in order to enter the highest degree of heaven.)
From American Atheists' press release:
"... Mormonism ... has already used its money and might to impose its beliefs on the non-Mormon citizens of California."
"We need to know if Mr. Romney supports these and other discriminatory actions of his church, for which he evangelized when it was still overtly racist, and to which he continues to donate millions of dollars."
...
"When President Kennedy was running for office ... he answered, in no uncertain terms, that he stood by this country and supported the separation of religion and government. All we are asking is that Mr. Romney address the same issue—as president, will he give his first priority to the U.S. Constitution or the Book of Mormon?"
Friendly Atheist reported that "Unlike their other billboards, this one will be featured on a mobile ad truck, which will drive around Lynn University (the site of the debate) tonight and then follow the Romney campaign on the road for the next few days."
It remains to be seen how the general public reacts to this instance of insisting that a religion not get a free pass....