It keeps the term "marriage" to mean a man and a woman, that's all. Prop 8 has no effect on civil unions. Cohabitating couples can still live together. Couples can still get civil unions. Couples still have visitation rights, insurance rights, etc, just like a marrage. Except, it is not called a marriage. That is all. Nobody is "breaking up" anything.
People in your country once used the same sort of rhetoric to defend having doctors' offices with a door for whites and a door for coloureds. It'd be the same service, just different.
The fact of the matter is that marriage is something people world-wide regard as special and important. It has ritual and implications far beyond the legal rights it grants. It carries this importance and tradition for people of a huge range of nationalities, races, and faiths (or lack thereof). Civil Unions have all the gravity and importance of registering a dog. It's wrong to think that denying gay people the right to marriage is of no consequence because civil unions provide the same service, just different.