I won't be surprised if this makes it way to SCOTUS. There are four votes guaranteed to strike down Prop 8, but I don't know how Kennedy will swing.
My hope is they'll use the Fourteenth to strike it down, then apply that same argument to the Fifth to strike down DOMA.
Loving v Virgina, a decision that ruled that state bans on inter-racial marriages are unconstitutional, will probably be used as precedent:
Marriage is one of the "basic civil rights of man," fundamental to our very existence and survival.... To deny this fundamental freedom on so unsupportable a basis as the racial classifications embodied in these statutes, classifications so directly subversive of the principle of equality at the heart of the Fourteenth Amendment, is surely to deprive all the State's citizens of liberty without due process of law. The Fourteenth Amendment requires that the freedom of choice to marry not be restricted by invidious racial discrimination. Under our Constitution, the freedom to marry, or not marry, a person of another race resides with the individual and cannot be infringed by the State.
FindLaw | Cases and Codes
And a partial list of legal protections afforded to legally-recognized marriage:
Legal and economic benefits of marriage
In 2009, the GAO prepared a new list which totaled about 1,100 federal benefits.
On the order of 1,400 legal rights are conferred upon married couples in the U.S. Typically these are composed of about 400 state benefits and over 1,000 federal benefits. Among them are the rights to:
joint parenting;
joint adoption;
joint foster care, custody, and visitation (including non-biological parents);
status as next-of-kin for hospital visits and medical decisions where one partner is too ill to be competent;
joint insurance policies for home, auto and health;
dissolution and divorce protections such as community property and child support;
immigration and residency for partners from other countries;
inheritance automatically in the absence of a will;
joint leases with automatic renewal rights in the event one partner dies or leaves the house or apartment;
inheritance of jointly-owned real and personal property through the right of survivorship (which avoids the time and expense and taxes in probate);
benefits such as annuities, pension plans, Social Security, and Medicare;
spousal exemptions to property tax increases upon the death of one partner who is a co-owner of the home;
veterans' discounts on medical care, education, and home loans; joint filing of tax returns;
joint filing of customs claims when traveling;
wrongful death benefits for a surviving partner and children;
bereavement or sick leave to care for a partner or child;
decision-making power with respect to whether a deceased partner will be cremated or not and where to bury him or her;
crime victims' recovery benefits;
loss of consortium tort benefits;
domestic violence protection orders;
judicial protections and evidentiary immunity
And finally, a fun fact: did you know that same-sex marriage was made legal in South Africa?