The ACLU explains the reasoning behind the lawsuit at its Web site:
The lawsuits allege that, on its face, Proposition 8 is an improper revision rather than an amendment of the California Constitution because, in its very title, which was "Eliminates the right to marry for same-sex couples," the initiative eliminated an existing right only for a targeted minority. If permitted to stand, Proposition 8 would be the first time an initiative has successfully been used to change the California Constitution to take away an existing right only for a particular group. Such a change would defeat the very purpose of a constitution and fundamentally alter the role of the courts in protecting minority rights. According to the California Constitution, such a serious revision of the state constitution cannot be enacted through a simple majority vote, but must first be approved by two-thirds of the legislature.
The article also notes the California Supreme Court's argument when it originally recognized the right of gays to marry:
In May of 2008, the California Supreme Court held that barring same-sex couples from marriage violates the equal protection clause of the California Constitution and violates the fundamental right to marry. Proposition 8 would completely eliminate the right to marry only for same-sex couples. No other initiative has ever successfully changed the California Constitution to take away a right only from a targeted minority group.
This entirely logical argument raises an important question: how is it that the population is allowed to vote on this issue and, through its decision, violate equal protection and the right to marry?
In the case Loving v. Virginia, 1967, the U.S. Supreme Court ruled that anti-miscegenation laws, which prevented interracial marriage, were unconstitutional. Would it not have been outrageous to subject such laws to a vote? Subjecting gay marriage rights to a vote is no less outrageous.